Air Force Declassification Office > Top Flight Documents

Unidentified flying object (commonly abbreviated as UFO or U.F.O.) is the popular term for any apparent aerial phenomenon whose cause cannot be easily or immediately identified by the observer. The United States Air Force, which coined the term in 1952, initially defined UFOs as those objects that remain unidentified after scrutiny by expert investigators, though today the term UFO is colloquially used to refer to any unidentifiable sighting regardless of whether it has been investigated. UFO reports increased precipitously after the first widely publicized U.S. sighting, reported by private pilot Kenneth Arnold in June 24 1947, that gave rise to the popular terms "flying saucer" and "flying disc." The term UFO is popularly taken as a synonym for alien spacecraft and generally most discussions of UFOs revolve around this presumption. UFO enthusiasts and devotees have created organizations, religious cults have adopted extraterrestrial themes, and in general the UFO concept has evolved into a prominent mythos in modern culture. Some investigators now prefer to use the broader term unidentified aerial phenomenon (or UAP), to avoid the confusion and speculative associations that have become attached to UFO. Another widely known acronym for UFO in Spanish, French, Portuguese and Italian is OVNI (Objeto Volador No Identificado, with variant regional spellings).