What are UFOs?

An unidentified flying object, or UFO, is the popular term for any aerial apparition that cannot immediately be identified. Some definitions, such as that which is used by the USAF, define a UFO as an object unable to be identified after scrutiny, while other definitions define an object as being a UFO from the time it is first reported as being unidentified, even though most subsequently become IFOs, Identified Flying Objects.

Reports of unusual aerial phenomena date back to ancient times, but modern reports and the first official investigations 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. Hundreds of thousands of UFO reports have since been made worldwide.

Since its introduction the term has become heavily associated with flying saucers and alien spacecraft, though an object may be classified as a UFO independently of opinion as to its origins. Most military and civilian UFO investigations concluded that the majority of objects can be identified either directly, or by applying Occam's Razor.