Caelum Incognitum — The interactive encyclopedia of ufology
UFO (unidentified flying object)
Classifications & scales
Any aerial object or phenomenon whose nature the witness could not establish at the time of the sighting. The term says nothing about extraterrestrial origin — it simply means “unidentified”. After investigation, the large majority of reports turn out to have a conventional explanation (aircraft, balloons, stars, satellites…). The acronym was popularized in the early 1950s by US Air Force Captain Edward J. Ruppelt.