Photograph of an alleged UFO in Passaic, New Jersey, taken on July 31, 1952.

An unidentified flying object (UFO) is an object observed in the sky that is not readily identified. Most UFOs are later identified as conventional objects or phenomena. The term is widely used for claimed observations of extraterrestrial spacecraft.


The term "UFO" (or "UFOB") was coined in 1953 by the United States Air Force (USAF) to serve as a catch-all for all such reports. In its initial definition, the USAF stated that a "UFOB" was "any airborne object which by performance, aerodynamic characteristics, or unusual features, does not conform to any presently known aircraft or missile type, or which cannot be positively identified as a familiar object." Accordingly, the term was initially restricted to that fraction of cases which remained unidentified after investigation, as the USAF was interested in potential national security reasons and "technical aspects" (see Air Force Regulation 200-2).

During the late 1940s and through the 1950s, UFOs were often referred to popularly as "flying saucers" or "flying discs". The term UFO became more widespread during the 1950s, at first in technical literature, but later in popular use. UFOs garnered considerable interest during the Cold War, an era associated with a heightened concern for national security, and, more recently, in the 2010s, for unexplained reasons.[1][2] Nevertheless, various studies have concluded that the phenomenon does not represent a threat to national security, nor does it contain anything worthy of scientific pursuit (e.g., 1951 Flying Saucer Working Party, 1953 CIA Robertson Panel, USAF Project Blue Book, Condon Committee).

The Oxford English Dictionary defines a UFO as "An unidentified flying object; a 'flying saucer'." The first published book to use the word was authored by Donald E. Keyhoe.[3]

As an acronym, "UFO" was coined by Captain Edward J. Ruppelt, who headed Project Blue Book, then the USAF's official investigation of UFOs. He wrote, "Obviously the term 'flying saucer' is misleading when applied to objects of every conceivable shape and performance. For this reason the military prefers the more general, if less colorful, name: unidentified flying objects. UFO (pronounced Yoo-foe) for short."[4] Other phrases that were used officially and that predate the UFO acronym include "flying flapjack", "flying disc", "unexplained flying discs", and "unidentifiable object".[5][6][7]

The phrase "flying saucer" had gained widespread attention after the summer of 1947. On June 24, a civilian pilot named Kenneth Arnold reported seeing nine objects flying in formation near Mount Rainier. Arnold timed the sighting and estimated the speed of discs to be over 1,200 mph (1,931 km/h). At the time, he claimed he described the objects flying in a saucer-like fashion, leading to newspaper accounts of "flying saucers" and "flying discs".[8][9] UFOs were commonly referred to colloquially, as a "Bogey" by military personal and pilots during the cold war. The term "bogey" was originally used to report anomalies in radar blips, to indicate possible hostile forces that might be roaming in the area.[10]

In popular usage, the term UFO came to be used to refer to claims of alien spacecraft,[3] and because of the public and media ridicule associated with the topic, some ufologists and investigators prefer to use terms such as "unidentified aerial phenomenon" (UAP) or "anomalous phenomena", as in the title of the National Aviation Reporting Center on Anomalous Phenomena (NARCAP).[11] "Anomalous aerial vehicle" (AAV) or "unidentified aerial system" (UAS) are also sometimes used in a military aviation context to describe unidentified targets.[12]


Studies have established that the majority of UFO observations are misidentified conventional objects or natural phenomena—most commonly aircraft, balloons, noctilucent clouds, nacreous clouds, or astronomical objects such as meteors or bright planets with a small percentage even being hoaxes.[note 1] Between 5% and 20% of reported sightings are not explained, and therefore can be classified as unidentified in the strictest sense. While proponents of the extraterrestrial hypothesis (ETH) suggest that these unexplained reports are of alien spacecraft, the null hypothesis cannot be excluded that these reports are simply other more prosaic phenomena that cannot be identified due to lack of complete information or due to the necessary subjectivity of the reports. Instead of accepting the null hypothesis, UFO enthusiasts tend to engage in special pleading by offering outlandish, untested explanations for the validity of the ETH. These violate Occam's razor.[13]

Almost no scientific papers about UFOs have been published in peer-reviewed journals.[14] There was, in the past, some debate in the scientific community about whether any scientific investigation into UFO sightings is warranted with the general conclusion being that the phenomenon was not worthy of serious investigation except as a cultural artifact.[15][16][17][18][19][20][21] UFOs have been the subject of investigations by various governments who have provided extensive records related to the subject. Many of the most involved government-sponsored investigations ended after agencies concluded that there was no benefit to continued investigation.[22][23]

The void left by the lack of institutional or scientific study has given rise to independent researchers and fringe groups, including the National Investigations Committee on Aerial Phenomena (NICAP) in the mid-20th century and, more recently, the Mutual UFO Network (MUFON)[24] and the Center for UFO Studies (CUFOS).[25] The term "Ufology" is used to describe the collective efforts of those who study reports and associated evidence of unidentified flying objects.[26]

UFOs have become a prevalent theme in modern culture,[27] and the social phenomena have been the subject of academic research in sociology and psychology.[14]

Early history

Unexplained aerial observations have been reported throughout history. Some were undoubtedly astronomical in nature: comets, bright meteors, one or more of the five planets that can be readily seen with the naked eye, planetary conjunctions, or atmospheric optical phenomena such as parhelia and lenticular clouds. An example is Halley's Comet, which was recorded first by Chinese astronomers in 240 BC and possibly as early as 467 BC. Such sightings throughout history often were treated as supernatural portents, angels, or other religious omens. Some current-day UFO researchers have noticed similarities between some religious symbols in medieval paintings and UFO reports[28] though the canonical and symbolic character of such images is documented by art historians placing more conventional religious interpretations on such images.[29]