An unidentified flying object (UFO) is any perceived aerial phenomenon that cannot be immediately identified or explained. Upon investigation, most UFOs are identified as known objects or atmospheric phenomena, while a small number remain unexplained.

Scientists and skeptic organizations such as the Committee for Skeptical Inquiry have provided prosaic explanations for a large number of claimed UFOs being caused by natural phenomena, human technology, delusions, or hoaxes. Small but vocal groups of ufologists favour unconventional or pseudoscientific hypotheses, often claiming that UFOs are evidence of extraterrestrial intelligence. Beliefs surrounding UFOs have inspired parts of new religions.

While unusual sightings have been reported in the sky throughout history, UFOs became culturally prominent after World War II, escalating during the Space Age. The 20th century saw studies and investigations into UFO reports conducted by governments (such as Project Blue Book in the United States and Project Condign in the United Kingdom), as well as by organisations and individuals.