A flying object whose nature is unknown; especially, anything considered to have extraterrestrial origins.

1. An small explosive jet-propelled winged-robot plane, or guided missile that carries a bomb.
2. A small winged missile that is loaded with explosives, jet-propelled, and guided only by a gyroscopic robot device.

Any light or object seen in the sky whose immediate identity is not apparent.

Despite unsubstantiated claims, there is no evidence that UFO's are alien spacecraft. When investigated, the vast majority of sightings turn out to have been of natural or identifiable objects, notably bright stars and planets, meteors, aircraft, and satellites, or to have been perpetrated by pranksters.