Unidentified flying object (commonly abbreviated UFO) is a popular term for any phenomenon seen in the sky that is not readily identified. The US AIR FORCE defines UFOs as objects remaining unidentified after scrutiny by expert investigators. Colloquially the term UFO is used to describe an as-yet unidentified sighting.
In popular culture the term UFO is often used as a synonym for alien spacecraft. Because of these different meanings that have become associated with UFO, some investigators now prefer to use the broader term Unidentified Aerial Phenomena (or UAP).
Some studies have established that a small percentage of reported UFOs are hoaxes. The majority of reports indicate something real, perhaps appearing anomalous, but most of these represent honest misidentifications of conventional objects such as aircraft, balloons, or astronomical objects such as meteors or bright planets. This leaves a small percentage of reported sightings, usually between around 5-15%, classified as unidentified 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"). Since then, millions of people believe they have seen UFOs and tens of thousands of such reports have been cataloged.
