An unidentified flying object (UFO) is any aerial phenomenon that cannot immediately be identified or explained. Most UFOs are identified on investigation as conventional objects or phenomena. The term is widely used for claimed observations of extraterrestrial spacecraft. Extraterrestrial life is hypothetical life which may occur outside of Earth and which did not originate on Earth. Such life might range from simple prokaryotes (or comparable life forms) to beings with civilizations far more advanced than humanity. The Drake equation speculates about the existence of intelligent life elsewhere in the universe.