From Borderland Sciences Research

An unidentified flying object (abbreviated UFO, or U.F.O.) is the general descriptor for any unusual object (or phenomenon) sighted in the sky that cannot be readily identified by an observer. In the majority of cases, a UFO can be later identified and explained to have been a known phenomenon or more conventional object, such as an aircraft, cloud feature, or planetary body, while the few that cannot be identified after the fact remain of interest to ufologists. A multitude of theories to explain the origins of unidentified craft exist, including the popular extraterrestrial hypothesis and the interdimensional hypothesis.

In popular media, the term UFO will typically refer to an extraterrestrial spacecraft, as routinely depicted in television and film, which, in turn, has inspired hoaxes and media inspired sighting reports.