Unidentified Aerial Phenomena

An unidentified flying object, or UFO, is in its strictest definition any apparent object in the sky that is not identifiable as a known object or phenomenon. However the term is widely used in popular culture to refer more specifically to supposed observations of craft of extraterrestrial origin.

Most UFOs are later identified as conventional objects or phenomena (e.g., aircraft, weather balloons, clouds). Some of them are not identified, either because of lack of evidence or because no conventional explanation can be found despite extensive evidence. Some people consider the latter cases as possible observations of extraterrestrial space craft.

The term "UFO" (or "UFOB") was officially created 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/or "technical aspects" (see ).

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. 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.

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