UFO is an
acronym for
Unidentified Flying Object. As such, in a literal sense it would include any object in the air of which the nature is not known, but usually when the term 'UFO' is used, it connotes the belief that many people have that at least some of the unidentified objects are actually starships of extraterrestrial
Aliens.
A good page for images of UFOs and the paranormal:
See also: Hollow Earth