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All Rights Reserved. * * Licensed under the W3C SOFTWARE AND DOCUMENT NOTICE AND LICENSE. * * https://www.w3.org/Consortium/Legal/2015/copyright-software-and-document * */ (function() { 'use strict'; // Exit early if we're not running in a browser. if (typeof window !== 'object') { return; } // Exit early if all IntersectionObserver and IntersectionObserverEntry // features are natively supported. if ('IntersectionObserver' in window && 'IntersectionObserverEntry' in window && 'intersectionRatio' in window.IntersectionObserverEntry.prototype) { // Minimal polyfill for Edge 15's lack of `isIntersecting` // See: https://github.com/w3c/IntersectionObserver/issues/211 if (!('isIntersecting' in window.IntersectionObserverEntry.prototype)) { Object.defineProperty(window.IntersectionObserverEntry.prototype, 'isIntersecting', { get: function () { return this.intersectionRatio > 0; } }); } return; } /** * Returns the embedding frame element, if any. * @param {!Document} doc * @return {!Element} */ function getFrameElement(doc) { try { return doc.defaultView && doc.defaultView.frameElement || null; } catch (e) { // Ignore the error. return null; } } /** * A local reference to the root document. */ var document = (function(startDoc) { var doc = startDoc; var frame = getFrameElement(doc); while (frame) { doc = frame.ownerDocument; frame = getFrameElement(doc); } return doc; })(window.document); /** * An IntersectionObserver registry. This registry exists to hold a strong * reference to IntersectionObserver instances currently observing a target * element. Without this registry, instances without another reference may be * garbage collected. */ var registry = []; /** * The signal updater for cross-origin intersection. When not null, it means * that the polyfill is configured to work in a cross-origin mode. * @type {function(DOMRect|ClientRect, DOMRect|ClientRect)} */ var crossOriginUpdater = null; /** * The current cross-origin intersection. Only used in the cross-origin mode. * @type {DOMRect|ClientRect} */ var crossOriginRect = null; /** * Creates the global IntersectionObserverEntry constructor. * https://w3c.github.io/IntersectionObserver/#intersection-observer-entry * @param {Object} entry A dictionary of instance properties. * @constructor */ function IntersectionObserverEntry(entry) { this.time = entry.time; this.target = entry.target; this.rootBounds = ensureDOMRect(entry.rootBounds); this.boundingClientRect = ensureDOMRect(entry.boundingClientRect); this.intersectionRect = ensureDOMRect(entry.intersectionRect || getEmptyRect()); this.isIntersecting = !!entry.intersectionRect; // Calculates the intersection ratio. var targetRect = this.boundingClientRect; var targetArea = targetRect.width * targetRect.height; var intersectionRect = this.intersectionRect; var intersectionArea = intersectionRect.width * intersectionRect.height; // Sets intersection ratio. if (targetArea) { // Round the intersection ratio to avoid floating point math issues: // https://github.com/w3c/IntersectionObserver/issues/324 this.intersectionRatio = Number((intersectionArea / targetArea).toFixed(4)); } else { // If area is zero and is intersecting, sets to 1, otherwise to 0 this.intersectionRatio = this.isIntersecting ? 1 : 0; } } /** * Creates the global IntersectionObserver constructor. * https://w3c.github.io/IntersectionObserver/#intersection-observer-interface * @param {Function} callback The function to be invoked after intersection * changes have queued. The function is not invoked if the queue has * been emptied by calling the `takeRecords` method. * @param {Object=} opt_options Optional configuration options. * @constructor */ function IntersectionObserver(callback, opt_options) { var options = opt_options || {}; if (typeof callback != 'function') { throw new Error('callback must be a function'); } if ( options.root && options.root.nodeType != 1 && options.root.nodeType != 9 ) { throw new Error('root must be a Document or Element'); } // Binds and throttles `this._checkForIntersections`. this._checkForIntersections = throttle( this._checkForIntersections.bind(this), this.THROTTLE_TIMEOUT); // Private properties. this._callback = callback; this._observationTargets = []; this._queuedEntries = []; this._rootMarginValues = this._parseRootMargin(options.rootMargin); // Public properties. this.thresholds = this._initThresholds(options.threshold); this.root = options.root || null; this.rootMargin = this._rootMarginValues.map(function(margin) { return margin.value + margin.unit; }).join(' '); /** @private @const {!Array<!Document>} */ this._monitoringDocuments = []; /** @private @const {!Array<function()>} */ this._monitoringUnsubscribes = []; } /** * The minimum interval within which the document will be checked for * intersection changes. */ IntersectionObserver.prototype.THROTTLE_TIMEOUT = 100; /** * The frequency in which the polyfill polls for intersection changes. * this can be updated on a per instance basis and must be set prior to * calling `observe` on the first target. */ IntersectionObserver.prototype.POLL_INTERVAL = null; /** * Use a mutation observer on the root element * to detect intersection changes. */ IntersectionObserver.prototype.USE_MUTATION_OBSERVER = true; /** * Sets up the polyfill in the cross-origin mode. The result is the * updater function that accepts two arguments: `boundingClientRect` and * `intersectionRect` - just as these fields would be available to the * parent via `IntersectionObserverEntry`. This function should be called * each time the iframe receives intersection information from the parent * window, e.g. via messaging. * @return {function(DOMRect|ClientRect, DOMRect|ClientRect)} */ IntersectionObserver._setupCrossOriginUpdater = function() { if (!crossOriginUpdater) { /** * @param {DOMRect|ClientRect} boundingClientRect * @param {DOMRect|ClientRect} intersectionRect */ crossOriginUpdater = function(boundingClientRect, intersectionRect) { if (!boundingClientRect || !intersectionRect) { crossOriginRect = getEmptyRect(); } else { crossOriginRect = convertFromParentRect(boundingClientRect, intersectionRect); } registry.forEach(function(observer) { observer._checkForIntersections(); }); }; } return crossOriginUpdater; }; /** * Resets the cross-origin mode. */ IntersectionObserver._resetCrossOriginUpdater = function() { crossOriginUpdater = null; crossOriginRect = null; }; /** * Starts observing a target element for intersection changes based on * the thresholds values. * @param {Element} target The DOM element to observe. */ IntersectionObserver.prototype.observe = function(target) { var isTargetAlreadyObserved = this._observationTargets.some(function(item) { return item.element == target; }); if (isTargetAlreadyObserved) { return; } if (!(target && target.nodeType == 1)) { throw new Error('target must be an Element'); } this._registerInstance(); this._observationTargets.push({element: target, entry: null}); this._monitorIntersections(target.ownerDocument); this._checkForIntersections(); }; /** * Stops observing a target element for intersection changes. * @param {Element} target The DOM element to observe. */ IntersectionObserver.prototype.unobserve = function(target) { this._observationTargets = this._observationTargets.filter(function(item) { return item.element != target; }); this._unmonitorIntersections(target.ownerDocument); if (this._observationTargets.length == 0) { this._unregisterInstance(); } }; /** * Stops observing all target elements for intersection changes. */ IntersectionObserver.prototype.disconnect = function() { this._observationTargets = []; this._unmonitorAllIntersections(); this._unregisterInstance(); }; /** * Returns any queue entries that have not yet been reported to the * callback and clears the queue. This can be used in conjunction with the * callback to obtain the absolute most up-to-date intersection information. * @return {Array} The currently queued entries. */ IntersectionObserver.prototype.takeRecords = function() { var records = this._queuedEntries.slice(); this._queuedEntries = []; return records; }; /** * Accepts the threshold value from the user configuration object and * returns a sorted array of unique threshold values. If a value is not * between 0 and 1 and error is thrown. * @private * @param {Array|number=} opt_threshold An optional threshold value or * a list of threshold values, defaulting to [0]. * @return {Array} A sorted list of unique and valid threshold values. */ IntersectionObserver.prototype._initThresholds = function(opt_threshold) { var threshold = opt_threshold || [0]; if (!Array.isArray(threshold)) threshold = [threshold]; return threshold.sort().filter(function(t, i, a) { if (typeof t != 'number' || isNaN(t) || t < 0 || t > 1) { throw new Error('threshold must be a number between 0 and 1 inclusively'); } return t !== a[i - 1]; }); }; /** * Accepts the rootMargin value from the user configuration object * and returns an array of the four margin values as an object containing * the value and unit properties. If any of the values are not properly * formatted or use a unit other than px or %, and error is thrown. * @private * @param {string=} opt_rootMargin An optional rootMargin value, * defaulting to '0px'. * @return {Array<Object>} An array of margin objects with the keys * value and unit. */ IntersectionObserver.prototype._parseRootMargin = function(opt_rootMargin) { var marginString = opt_rootMargin || '0px'; var margins = marginString.split(/\s+/).map(function(margin) { var parts = /^(-?\d*\.?\d+)(px|%)$/.exec(margin); if (!parts) { throw new Error('rootMargin must be specified in pixels or percent'); } return {value: parseFloat(parts[1]), unit: parts[2]}; }); // Handles shorthand. margins[1] = margins[1] || margins[0]; margins[2] = margins[2] || margins[0]; margins[3] = margins[3] || margins[1]; return margins; }; /** * Starts polling for intersection changes if the polling is not already * happening, and if the page's visibility state is visible. * @param {!Document} doc * @private */ IntersectionObserver.prototype._monitorIntersections = function(doc) { var win = doc.defaultView; if (!win) { // Already destroyed. return; } if (this._monitoringDocuments.indexOf(doc) != -1) { // Already monitoring. return; } // Private state for monitoring. var callback = this._checkForIntersections; var monitoringInterval = null; var domObserver = null; // If a poll interval is set, use polling instead of listening to // resize and scroll events or DOM mutations. if (this.POLL_INTERVAL) { monitoringInterval = win.setInterval(callback, this.POLL_INTERVAL); } else { addEvent(win, 'resize', callback, true); addEvent(doc, 'scroll', callback, true); if (this.USE_MUTATION_OBSERVER && 'MutationObserver' in win) { domObserver = new win.MutationObserver(callback); domObserver.observe(doc, { attributes: true, childList: true, characterData: true, subtree: true }); } } this._monitoringDocuments.push(doc); this._monitoringUnsubscribes.push(function() { // Get the window object again. When a friendly iframe is destroyed, it // will be null. var win = doc.defaultView; if (win) { if (monitoringInterval) { win.clearInterval(monitoringInterval); } removeEvent(win, 'resize', callback, true); } removeEvent(doc, 'scroll', callback, true); if (domObserver) { domObserver.disconnect(); } }); // Also monitor the parent. var rootDoc = (this.root && (this.root.ownerDocument || this.root)) || document; if (doc != rootDoc) { var frame = getFrameElement(doc); if (frame) { this._monitorIntersections(frame.ownerDocument); } } }; /** * Stops polling for intersection changes. * @param {!Document} doc * @private */ IntersectionObserver.prototype._unmonitorIntersections = function(doc) { var index = this._monitoringDocuments.indexOf(doc); if (index == -1) { return; } var rootDoc = (this.root && (this.root.ownerDocument || this.root)) || document; // Check if any dependent targets are still remaining. var hasDependentTargets = this._observationTargets.some(function(item) { var itemDoc = item.element.ownerDocument; // Target is in this context. if (itemDoc == doc) { return true; } // Target is nested in this context. while (itemDoc && itemDoc != rootDoc) { var frame = getFrameElement(itemDoc); itemDoc = frame && frame.ownerDocument; if (itemDoc == doc) { return true; } } return false; }); if (hasDependentTargets) { return; } // Unsubscribe. var unsubscribe = this._monitoringUnsubscribes[index]; this._monitoringDocuments.splice(index, 1); this._monitoringUnsubscribes.splice(index, 1); unsubscribe(); // Also unmonitor the parent. if (doc != rootDoc) { var frame = getFrameElement(doc); if (frame) { this._unmonitorIntersections(frame.ownerDocument); } } }; /** * Stops polling for intersection changes. * @param {!Document} doc * @private */ IntersectionObserver.prototype._unmonitorAllIntersections = function() { var unsubscribes = this._monitoringUnsubscribes.slice(0); this._monitoringDocuments.length = 0; this._monitoringUnsubscribes.length = 0; for (var i = 0; i < unsubscribes.length; i++) { unsubscribes[i](); } }; /** * Scans each observation target for intersection changes and adds them * to the internal entries queue. If new entries are found, it * schedules the callback to be invoked. * @private */ IntersectionObserver.prototype._checkForIntersections = function() { if (!this.root && crossOriginUpdater && !crossOriginRect) { // Cross origin monitoring, but no initial data available yet. return; } var rootIsInDom = this._rootIsInDom(); var rootRect = rootIsInDom ? this._getRootRect() : getEmptyRect(); this._observationTargets.forEach(function(item) { var target = item.element; var targetRect = getBoundingClientRect(target); var rootContainsTarget = this._rootContainsTarget(target); var oldEntry = item.entry; var intersectionRect = rootIsInDom && rootContainsTarget && this._computeTargetAndRootIntersection(target, targetRect, rootRect); var rootBounds = null; if (!this._rootContainsTarget(target)) { rootBounds = getEmptyRect(); } else if (!crossOriginUpdater || this.root) { rootBounds = rootRect; } var newEntry = item.entry = new IntersectionObserverEntry({ time: now(), target: target, boundingClientRect: targetRect, rootBounds: rootBounds, intersectionRect: intersectionRect }); if (!oldEntry) { this._queuedEntries.push(newEntry); } else if (rootIsInDom && rootContainsTarget) { // If the new entry intersection ratio has crossed any of the // thresholds, add a new entry. if (this._hasCrossedThreshold(oldEntry, newEntry)) { this._queuedEntries.push(newEntry); } } else { // If the root is not in the DOM or target is not contained within // root but the previous entry for this target had an intersection, // add a new record indicating removal. if (oldEntry && oldEntry.isIntersecting) { this._queuedEntries.push(newEntry); } } }, this); if (this._queuedEntries.length) { this._callback(this.takeRecords(), this); } }; /** * Accepts a target and root rect computes the intersection between then * following the algorithm in the spec. * TODO(philipwalton): at this time clip-path is not considered. * https://w3c.github.io/IntersectionObserver/#calculate-intersection-rect-algo * @param {Element} target The target DOM element * @param {Object} targetRect The bounding rect of the target. * @param {Object} rootRect The bounding rect of the root after being * expanded by the rootMargin value. * @return {?Object} The final intersection rect object or undefined if no * intersection is found. * @private */ IntersectionObserver.prototype._computeTargetAndRootIntersection = function(target, targetRect, rootRect) { // If the element isn't displayed, an intersection can't happen. if (window.getComputedStyle(target).display == 'none') return; var intersectionRect = targetRect; var parent = getParentNode(target); var atRoot = false; while (!atRoot && parent) { var parentRect = null; var parentComputedStyle = parent.nodeType == 1 ? window.getComputedStyle(parent) : {}; // If the parent isn't displayed, an intersection can't happen. if (parentComputedStyle.display == 'none') return null; if (parent == this.root || parent.nodeType == /* DOCUMENT */ 9) { atRoot = true; if (parent == this.root || parent == document) { if (crossOriginUpdater && !this.root) { if (!crossOriginRect || crossOriginRect.width == 0 && crossOriginRect.height == 0) { // A 0-size cross-origin intersection means no-intersection. parent = null; parentRect = null; intersectionRect = null; } else { parentRect = crossOriginRect; } } else { parentRect = rootRect; } } else { // Check if there's a frame that can be navigated to. var frame = getParentNode(parent); var frameRect = frame && getBoundingClientRect(frame); var frameIntersect = frame && this._computeTargetAndRootIntersection(frame, frameRect, rootRect); if (frameRect && frameIntersect) { parent = frame; parentRect = convertFromParentRect(frameRect, frameIntersect); } else { parent = null; intersectionRect = null; } } } else { // If the element has a non-visible overflow, and it's not the <body> // or <html> element, update the intersection rect. // Note: <body> and <html> cannot be clipped to a rect that's not also // the document rect, so no need to compute a new intersection. var doc = parent.ownerDocument; if (parent != doc.body && parent != doc.documentElement && parentComputedStyle.overflow != 'visible') { parentRect = getBoundingClientRect(parent); } } // If either of the above conditionals set a new parentRect, // calculate new intersection data. if (parentRect) { intersectionRect = computeRectIntersection(parentRect, intersectionRect); } if (!intersectionRect) break; parent = parent && getParentNode(parent); } return intersectionRect; }; /** * Returns the root rect after being expanded by the rootMargin value. * @return {ClientRect} The expanded root rect. * @private */ IntersectionObserver.prototype._getRootRect = function() { var rootRect; if (this.root && !isDoc(this.root)) { rootRect = getBoundingClientRect(this.root); } else { // Use <html>/<body> instead of window since scroll bars affect size. var doc = isDoc(this.root) ? this.root : document; var html = doc.documentElement; var body = doc.body; rootRect = { top: 0, left: 0, right: html.clientWidth || body.clientWidth, width: html.clientWidth || body.clientWidth, bottom: html.clientHeight || body.clientHeight, height: html.clientHeight || body.clientHeight }; } return this._expandRectByRootMargin(rootRect); }; /** * Accepts a rect and expands it by the rootMargin value. * @param {DOMRect|ClientRect} rect The rect object to expand. * @return {ClientRect} The expanded rect. * @private */ IntersectionObserver.prototype._expandRectByRootMargin = function(rect) { var margins = this._rootMarginValues.map(function(margin, i) { return margin.unit == 'px' ? margin.value : margin.value * (i % 2 ? rect.width : rect.height) / 100; }); var newRect = { top: rect.top - margins[0], right: rect.right + margins[1], bottom: rect.bottom + margins[2], left: rect.left - margins[3] }; newRect.width = newRect.right - newRect.left; newRect.height = newRect.bottom - newRect.top; return newRect; }; /** * Accepts an old and new entry and returns true if at least one of the * threshold values has been crossed. * @param {?IntersectionObserverEntry} oldEntry The previous entry for a * particular target element or null if no previous entry exists. * @param {IntersectionObserverEntry} newEntry The current entry for a * particular target element. * @return {boolean} Returns true if a any threshold has been crossed. * @private */ IntersectionObserver.prototype._hasCrossedThreshold = function(oldEntry, newEntry) { // To make comparing easier, an entry that has a ratio of 0 // but does not actually intersect is given a value of -1 var oldRatio = oldEntry && oldEntry.isIntersecting ? oldEntry.intersectionRatio || 0 : -1; var newRatio = newEntry.isIntersecting ? newEntry.intersectionRatio || 0 : -1; // Ignore unchanged ratios if (oldRatio === newRatio) return; for (var i = 0; i < this.thresholds.length; i++) { var threshold = this.thresholds[i]; // Return true if an entry matches a threshold or if the new ratio // and the old ratio are on the opposite sides of a threshold. if (threshold == oldRatio || threshold == newRatio || threshold < oldRatio !== threshold < newRatio) { return true; } } }; /** * Returns whether or not the root element is an element and is in the DOM. * @return {boolean} True if the root element is an element and is in the DOM. * @private */ IntersectionObserver.prototype._rootIsInDom = function() { return !this.root || containsDeep(document, this.root); }; /** * Returns whether or not the target element is a child of root. * @param {Element} target The target element to check. * @return {boolean} True if the target element is a child of root. * @private */ IntersectionObserver.prototype._rootContainsTarget = function(target) { var rootDoc = (this.root && (this.root.ownerDocument || this.root)) || document; return ( containsDeep(rootDoc, target) && (!this.root || rootDoc == target.ownerDocument) ); }; /** * Adds the instance to the global IntersectionObserver registry if it isn't * already present. * @private */ IntersectionObserver.prototype._registerInstance = function() { if (registry.indexOf(this) < 0) { registry.push(this); } }; /** * Removes the instance from the global IntersectionObserver registry. * @private */ IntersectionObserver.prototype._unregisterInstance = function() { var index = registry.indexOf(this); if (index != -1) registry.splice(index, 1); }; /** * Returns the result of the performance.now() method or null in browsers * that don't support the API. * @return {number} The elapsed time since the page was requested. */ function now() { return window.performance && performance.now && performance.now(); } /** * Throttles a function and delays its execution, so it's only called at most * once within a given time period. * @param {Function} fn The function to throttle. * @param {number} timeout The amount of time that must pass before the * function can be called again. * @return {Function} The throttled function. */ function throttle(fn, timeout) { var timer = null; return function () { if (!timer) { timer = setTimeout(function() { fn(); timer = null; }, timeout); } }; } /** * Adds an event handler to a DOM node ensuring cross-browser compatibility. * @param {Node} node The DOM node to add the event handler to. * @param {string} event The event name. * @param {Function} fn The event handler to add. * @param {boolean} opt_useCapture Optionally adds the even to the capture * phase. Note: this only works in modern browsers. */ function addEvent(node, event, fn, opt_useCapture) { if (typeof node.addEventListener == 'function') { node.addEventListener(event, fn, opt_useCapture || false); } else if (typeof node.attachEvent == 'function') { node.attachEvent('on' + event, fn); } } /** * Removes a previously added event handler from a DOM node. * @param {Node} node The DOM node to remove the event handler from. * @param {string} event The event name. * @param {Function} fn The event handler to remove. * @param {boolean} opt_useCapture If the event handler was added with this * flag set to true, it should be set to true here in order to remove it. */ function removeEvent(node, event, fn, opt_useCapture) { if (typeof node.removeEventListener == 'function') { node.removeEventListener(event, fn, opt_useCapture || false); } else if (typeof node.detatchEvent == 'function') { node.detatchEvent('on' + event, fn); } } /** * Returns the intersection between two rect objects. * @param {Object} rect1 The first rect. * @param {Object} rect2 The second rect. * @return {?Object|?ClientRect} The intersection rect or undefined if no * intersection is found. */ function computeRectIntersection(rect1, rect2) { var top = Math.max(rect1.top, rect2.top); var bottom = Math.min(rect1.bottom, rect2.bottom); var left = Math.max(rect1.left, rect2.left); var right = Math.min(rect1.right, rect2.right); var width = right - left; var height = bottom - top; return (width >= 0 && height >= 0) && { top: top, bottom: bottom, left: left, right: right, width: width, height: height } || null; } /** * Shims the native getBoundingClientRect for compatibility with older IE. * @param {Element} el The element whose bounding rect to get. * @return {DOMRect|ClientRect} The (possibly shimmed) rect of the element. */ function getBoundingClientRect(el) { var rect; try { rect = el.getBoundingClientRect(); } catch (err) { // Ignore Windows 7 IE11 "Unspecified error" // https://github.com/w3c/IntersectionObserver/pull/205 } if (!rect) return getEmptyRect(); // Older IE if (!(rect.width && rect.height)) { rect = { top: rect.top, right: rect.right, bottom: rect.bottom, left: rect.left, width: rect.right - rect.left, height: rect.bottom - rect.top }; } return rect; } /** * Returns an empty rect object. An empty rect is returned when an element * is not in the DOM. * @return {ClientRect} The empty rect. */ function getEmptyRect() { return { top: 0, bottom: 0, left: 0, right: 0, width: 0, height: 0 }; } /** * Ensure that the result has all of the necessary fields of the DOMRect. * Specifically this ensures that `x` and `y` fields are set. * * @param {?DOMRect|?ClientRect} rect * @return {?DOMRect} */ function ensureDOMRect(rect) { // A `DOMRect` object has `x` and `y` fields. if (!rect || 'x' in rect) { return rect; } // A IE's `ClientRect` type does not have `x` and `y`. The same is the case // for internally calculated Rect objects. 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UFO","wordcount":"466","publisher":{"@type":"Organization","name":"Lexico.com","logo":{"@type":"ImageObject","url":"https://web.archive.org/web/20211112070049/https://www.lexico.com/lexico-logo.svg"}},"url":"https://web.archive.org/web/20211112070049/https://lexico.com","mainEntityOfPage":{"@type":"WebPage","@id":"https://web.archive.org/web/20211112070049/https://lexico.com/explore"},"datePublished":"2019-03-26","dateCreated":"2019-03-26","dateModified":"2019-12-09","description":"There are many theories about the existence of UFOs, but where did this initialism come from? Watch our video to find out the real truth.","articleBody":"\u003cp\u003e\u003cspan class=\"fr-embedcode fr-dvb\" contenteditable=\"false\"\u003e\u003ciframe src=\"https://players.brightcove.net/5660549791001/default_default/index.html?videoId=6083825004001\" allowfullscreen=\"\" allow=\"encrypted-media\" width=\"480\" height=\"270\"\u003e\u003c/iframe\u003e\u003c/span\u003e\u003cbr\u003e\u003c/p\u003e\u003cp\u003eThe word \u003cem\u003eUFO\u003c/em\u003e is an \u003ca href=\"/definition/abbreviation\"\u003eabbreviation\u003c/a\u003e which comes from the initial letters of the words ‘unidentified flying object’. A UFO is a \u003ca href=\"/definition/mysterious\"\u003emysterious\u003c/a\u003e object in the sky which can’t be explained by current scientific theories. UFOs are often associated with \u003ca href=\"/definition/extraterrestrial\"\u003eextraterrestrial\u003c/a\u003e beings: they’re believed to be the spaceships of the ‘little green men’ of \u003ca href=\"/definition/science_fiction\"\u003escience-fiction\u003c/a\u003e movies and stories.\u003c/p\u003e\u003cp\u003eAccording to the \u003cem\u003eOxford English Dictionary\u003c/em\u003e, the term ‘unidentified flying object’ goes back at least to the 1950s: it is recorded in 1953, in a book by the US aviator and writer Donald Keyhoe. The \u003cem\u003eOED\u003c/em\u003e also gives a citation in 1956 by Edward Ruppelt, an officer in the \u003ca href=\"/definition/usaf\"\u003eUSAF\u003c/a\u003e, stating that he’d invented the term as a more general one to replace the earlier description for such objects, \u003ca href=\"/definition/flying_saucer\"\u003eflying saucer\u003c/a\u003e.\u003c/p\u003e\u003cp\u003eStrange objects in the sky were first named \u003cem\u003eflying saucers\u003c/em\u003e in the 1940s: the first \u003cem\u003eOED\u003c/em\u003e citation is from \u003cem\u003eThe Times\u003c/em\u003e, in 1947. They were so called because of an account by a US pilot, Kenneth Arnold, who stated in various newspaper and radio interviews of that year that he’d seen ‘saucer-like’ objects in the sky while he was flying past Mount Rainier. The media seemed to have quickly changed this to the \u003ca href=\"/definition/snappy#snappy__7\"\u003esnappier\u003c/a\u003e ‘flying saucer’ and so the term was born. By the time Ruppelt and his USAF colleagues were investigating reports of these sightings in the 1950s, it was clear that ‘saucer’ was too limited a description, since the objects in question were said to be of many different shapes: hence Ruppelt’s invention of ‘UFO’.\u003c/p\u003e\u003cp\u003eUFOs captured the public’s imagination and it wasn’t long before sightings were reported from all over the world. The study of UFOs became known as \u003ca href=\"/definition/ufology\"\u003eufology\u003c/a\u003e (recorded in \u003cem\u003eThe Times Literary Supplement\u003c/em\u003e in 1959) and experts in the field were named \u003ca href=\"/definition/ufology#ufology__8\"\u003eufologists\u003c/a\u003e about four years later, even though research in this area isn’t generally regarded as an academic discipline.\u003c/p\u003e\u003cp\u003e\u0026nbsp;\u003c/p\u003e\u003cp\u003eYou may also be interested in: \u003ca href=\"/explore/is-a-tomato-a-fruit-or-a-vegetable\"\u003eIs a tomato a fruit or a vegetable?\u003c/a\u003e\u003c/p\u003e\u003cp\u003eOr have a look at other similar queries in our \u003ca href=\"/explore/word-origins\"\u003eword origins\u003c/a\u003e section.\u003c/p\u003e"} //]]> </script><div id="content"><div class="lex-container"><div class="banbox mobile_banner"><div class="container content-ban"><aside class="banner adUnit"><div id="lexi_serp_dkt_atf_728x90_10"></div><div id="lexi_serp_mweb_atf_320x50_1"></div></aside></div></div><div class="main-content"><div class="container layout"><div class="lex-content"><div class="lex-filling"><div><div class="breadcrumbs layout"><p><a href="/web/20211112070049/https://www.lexico.com/" data-behaviour="ga-event-breadcrumb" data-value="Homepage anchor">Home</a><span class="chevron"></span> <a data-behaviour="ga-event" data-value="secondary breadcrumb" href="/web/20211112070049/https://www.lexico.com/explore">Explore The English Language</a> <span class="chevron"></span> <a data-behaviour="ga-event" data-value="secondary breadcrumb" href="/web/20211112070049/https://www.lexico.com/explore/word-origins">Word Origins</a> <span class="chevron"></span> <a data-behaviour="ga-event" data-value="secondary breadcrumb" href="/web/20211112070049/https://www.lexico.com/explore/what-is-the-origin-of-the-term-ufo">What Is The Origin Of The Term "UFO"?</a></p></div><div class="non-lexical-socials"><div class="socials"><ul><li><a class="ico-fb" href="https://web.archive.org/web/20211112070049/https://www.facebook.com/dialog/share?app_id=292327308384887&display=page&href=https%3A%2F%2Fwww.lexico.com%2Fexplore%2Fwhat-is-the-origin-of-the-term-ufo&redirect_uri=https%3A%2F%2Fwww.lexico.com%2Fexplore%2Fwhat-is-the-origin-of-the-term-ufo" target="_blank" rel="noopener" title="Share on Facebook" onclick="javascript:ga('send', 'event', 'share_this_entry', 'facebook', 'https://web.archive.org/web/20211112070049/https://www.lexico.com/explore/what-is-the-origin-of-the-term-ufo');" data-behaviour="ga-event-entry-sharing" data-value="Facebook"><i class="transition"><svg enable-background="new 0 0 30 30" viewbox="0 0 30 30" xmlns="http://www.w3.org/2000/svg"><path d="m15 0c-8.284 0-15 6.716-15 15 0 8.285 6.716 15 15 15 8.284 0 15-6.715 15-15 0-8.284-6.716-15-15-15m3.214 15.01h-2.102c0 3.357 0 7.495 0 7.495h-3.115c0 0 0-4.095 0-7.495h-1.482v-2.647h1.482v-1.714c0-1.227.582-3.144 3.143-3.144l2.307.009v2.571c0 0-1.404 0-1.676 0-.271 0-.66.136-.66.722v1.556h2.375l-.272 2.647" fill="#37589a"/></svg></i></a></li> <li><a class="ico-tw" href="https://web.archive.org/web/20211112070049/https://twitter.com/intent/tweet?text=What%20Is%20The%20Origin%20Of%20The%20Term%20%22UFO%22%3F%20-%20Lexico.com&url=https%3A%2F%2Fwww.lexico.com%2Fexplore%2Fwhat-is-the-origin-of-the-term-ufo" target="_blank" rel="noopener" title="Share on Twitter" onclick="javascript:ga('send', 'event', 'share_this_entry', 'twitter', 'https://web.archive.org/web/20211112070049/https://www.lexico.com/explore/what-is-the-origin-of-the-term-ufo');" data-behaviour="ga-event-entry-sharing" data-value="Twitter"><i class="transition"><svg enable-background="new 0 0 30 30" viewbox="0 0 30 30" xmlns="http://www.w3.org/2000/svg"><path d="m15 0c-8.284 0-15 6.716-15 15 0 8.285 6.716 15 15 15 8.283 0 15-6.715 15-15 0-8.284-6.717-15-15-15m5.965 11.943c.006.132.001.264.001.397 0 4.066-3.095 8.755-8.757 8.755-1.738 0-3.356-.509-4.718-1.382.242.028.486.041.734.041 1.443 0 2.769-.489 3.821-1.314-1.345-.025-2.483-.916-2.874-2.136.188.033.38.053.579.053.281 0 .553-.039.81-.107-1.407-.284-2.468-1.527-2.468-3.02 0-.013 0-.026 0-.039.416.231.891.368 1.395.384-.825-.551-1.369-1.494-1.369-2.562 0-.564.152-1.093.417-1.547 1.517 1.863 3.786 3.087 6.342 3.215-.052-.226-.08-.459-.08-.701 0-1.699 1.377-3.076 3.079-3.076.885 0 1.685.374 2.246.97.699-.137 1.359-.394 1.955-.748-.232.719-.719 1.322-1.354 1.703.624-.073 1.217-.24 1.768-.483-.413.618-.936 1.16-1.536 1.595" fill="#38a8e0"/></svg></i></a></li> <li><a class="ico-lin" href="https://web.archive.org/web/20211112070049/https://www.linkedin.com/cws/share?url=https%3A%2F%2Fwww.lexico.com%2Fexplore%2Fwhat-is-the-origin-of-the-term-ufo" target="_blank" rel="noopener" title="Share on Linkedin+" onclick="javascript:window.open(this.href,'', 'menubar=no,toolbar=no,resizable=yes,scrollbars=yes,height=600,width=600'); 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A UFO is a <a href="/web/20211112070049/https://www.lexico.com/definition/mysterious">mysterious</a> object in the sky which can’t be explained by current scientific theories. UFOs are often associated with <a href="/web/20211112070049/https://www.lexico.com/definition/extraterrestrial">extraterrestrial</a> beings: they’re believed to be the spaceships of the ‘little green men’ of <a href="/web/20211112070049/https://www.lexico.com/definition/science_fiction">science-fiction</a> movies and stories.</p><p>According to the <em>Oxford English Dictionary</em>, the term ‘unidentified flying object’ goes back at least to the 1950s: it is recorded in 1953, in a book by the US aviator and writer Donald Keyhoe. The <em>OED</em> also gives a citation in 1956 by Edward Ruppelt, an officer in the <a href="/web/20211112070049/https://www.lexico.com/definition/usaf">USAF</a>, stating that he’d invented the term as a more general one to replace the earlier description for such objects, <a href="/web/20211112070049/https://www.lexico.com/definition/flying_saucer">flying saucer</a>.</p><p>Strange objects in the sky were first named <em>flying saucers</em> in the 1940s: the first <em>OED</em> citation is from <em>The Times</em>, in 1947. They were so called because of an account by a US pilot, Kenneth Arnold, who stated in various newspaper and radio interviews of that year that he’d seen ‘saucer-like’ objects in the sky while he was flying past Mount Rainier. The media seemed to have quickly changed this to the <a href="/web/20211112070049/https://www.lexico.com/definition/snappy#snappy__7">snappier</a> ‘flying saucer’ and so the term was born. By the time Ruppelt and his USAF colleagues were investigating reports of these sightings in the 1950s, it was clear that ‘saucer’ was too limited a description, since the objects in question were said to be of many different shapes: hence Ruppelt’s invention of ‘UFO’.</p><p>UFOs captured the public’s imagination and it wasn’t long before sightings were reported from all over the world. The study of UFOs became known as <a href="/web/20211112070049/https://www.lexico.com/definition/ufology">ufology</a> (recorded in <em>The Times Literary Supplement</em> in 1959) and experts in the field were named <a href="/web/20211112070049/https://www.lexico.com/definition/ufology#ufology__8">ufologists</a> about four years later, even though research in this area isn’t generally regarded as an academic discipline.</p><p> </p><p>You may also be interested in: <a href="/web/20211112070049/https://www.lexico.com/explore/is-a-tomato-a-fruit-or-a-vegetable">Is a tomato a fruit or a vegetable?</a></p><p>Or have a look at other similar queries in our <a href="/web/20211112070049/https://www.lexico.com/explore/word-origins">word origins</a> section.</p></p><div class="non-lexical-socials socials-mobile"><div class="socials"><ul><li><a class="ico-fb" href="https://web.archive.org/web/20211112070049/https://www.facebook.com/dialog/share?app_id=292327308384887&display=page&href=https%3A%2F%2Fwww.lexico.com%2Fexplore%2Fwhat-is-the-origin-of-the-term-ufo&redirect_uri=https%3A%2F%2Fwww.lexico.com%2Fexplore%2Fwhat-is-the-origin-of-the-term-ufo" target="_blank" rel="noopener" title="Share on Facebook" onclick="javascript:ga('send', 'event', 'share_this_entry', 'facebook', 'https://web.archive.org/web/20211112070049/https://www.lexico.com/explore/what-is-the-origin-of-the-term-ufo');" data-behaviour="ga-event-entry-sharing" data-value="Facebook"><i class="transition"><svg enable-background="new 0 0 30 30" viewbox="0 0 30 30" xmlns="http://www.w3.org/2000/svg"><path d="m15 0c-8.284 0-15 6.716-15 15 0 8.285 6.716 15 15 15 8.284 0 15-6.715 15-15 0-8.284-6.716-15-15-15m3.214 15.01h-2.102c0 3.357 0 7.495 0 7.495h-3.115c0 0 0-4.095 0-7.495h-1.482v-2.647h1.482v-1.714c0-1.227.582-3.144 3.143-3.144l2.307.009v2.571c0 0-1.404 0-1.676 0-.271 0-.66.136-.66.722v1.556h2.375l-.272 2.647" fill="#37589a"/></svg></i></a></li> <li><a class="ico-tw" href="https://web.archive.org/web/20211112070049/https://twitter.com/intent/tweet?text=What%20Is%20The%20Origin%20Of%20The%20Term%20%22UFO%22%3F%20-%20Lexico.com&url=https%3A%2F%2Fwww.lexico.com%2Fexplore%2Fwhat-is-the-origin-of-the-term-ufo" target="_blank" rel="noopener" title="Share on Twitter" onclick="javascript:ga('send', 'event', 'share_this_entry', 'twitter', 'https://web.archive.org/web/20211112070049/https://www.lexico.com/explore/what-is-the-origin-of-the-term-ufo');" data-behaviour="ga-event-entry-sharing" data-value="Twitter"><i class="transition"><svg enable-background="new 0 0 30 30" viewbox="0 0 30 30" xmlns="http://www.w3.org/2000/svg"><path d="m15 0c-8.284 0-15 6.716-15 15 0 8.285 6.716 15 15 15 8.283 0 15-6.715 15-15 0-8.284-6.717-15-15-15m5.965 11.943c.006.132.001.264.001.397 0 4.066-3.095 8.755-8.757 8.755-1.738 0-3.356-.509-4.718-1.382.242.028.486.041.734.041 1.443 0 2.769-.489 3.821-1.314-1.345-.025-2.483-.916-2.874-2.136.188.033.38.053.579.053.281 0 .553-.039.81-.107-1.407-.284-2.468-1.527-2.468-3.02 0-.013 0-.026 0-.039.416.231.891.368 1.395.384-.825-.551-1.369-1.494-1.369-2.562 0-.564.152-1.093.417-1.547 1.517 1.863 3.786 3.087 6.342 3.215-.052-.226-.08-.459-.08-.701 0-1.699 1.377-3.076 3.079-3.076.885 0 1.685.374 2.246.97.699-.137 1.359-.394 1.955-.748-.232.719-.719 1.322-1.354 1.703.624-.073 1.217-.24 1.768-.483-.413.618-.936 1.16-1.536 1.595" fill="#38a8e0"/></svg></i></a></li> <li><a class="ico-lin" href="https://web.archive.org/web/20211112070049/https://www.linkedin.com/cws/share?url=https%3A%2F%2Fwww.lexico.com%2Fexplore%2Fwhat-is-the-origin-of-the-term-ufo" target="_blank" rel="noopener" title="Share on Linkedin+" onclick="javascript:window.open(this.href,'', 'menubar=no,toolbar=no,resizable=yes,scrollbars=yes,height=600,width=600'); 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