An Unidentified Flying Object (UFO) is the popular term for claims of aerial phenomenon that are not readily identified. Some definitions, such as the one used by the USAF, define UFOs as only those objects remaining unidentified after scrutiny by expert investigators, while other definitions define an object as being a UFO from the time it is first reported as being unidentified. However, many military and civilian investigations have concluded that UFOs can be identified either directly, or by applying Occam's Razor, and therefore are IFOs, Identified Flying Objects.
Modern reports and the first official investigations of UFOs began during World War II with sightings of so-called foo fighters by Allied airplane crews, and in 1946 with widespread sightings of European "ghost rockets". UFO reports became even more common after the first widely publicized United States UFO sighting, by private pilot Kenneth Arnold in mid 1947 (which gave rise to the popular terms "flying saucer" and "flying disc"). Hundreds of thousands people believe they have seen UFOs since then.

Despite introduction in 1952 by the USAF of the term "UFO" to replace "flying saucers", UFO is still sometimes used as a synonym for alien spacecraft in popular culture, though an anomaly may be classified as a UFO independently of opinion as to its origins. Some investigators prefer to use the broader term Unidentified Aerial Phenomena or UAP


参考文献：wikipedia