Unidentified flying object (commonly abbreviated as UFO or U.F.O.) is the popular term for any aerial phenomenon whose cause cannot be easily or immediately identified by the observer. The United States Air Force, which coined the term in 1952, initially defined UFOs as those objects that remain unidentified after scrutiny by expert investigators, though the term UFO is often used more generally to describe any sighting unidentifiable to the reporting observer(s). Popular culture frequently takes the term UFO as a synonym for alien spacecraft. Cults have become associated with UFOs, and mythology and folklore have evolved around the phenomenon. Some investigators now prefer to use the broader term unidentified aerial phenomenon (or UAP), to avoid the confusion and speculative associations that have become attached to UFO. Another widely known acronym for UFO in Spanish, French, Portuguese and Italian is OVNI.

Studies have established that the majority of UFOs are observations of some real but conventional object—most commonly aircraft, balloons, or astronomical objects such as meteors or bright planets—that have been misidentified by the observer as anomalies while a small percentage of reported UFOs are hoaxes. Only a small percentage of reported sightings (usually 5 to 20%) can be classified as unidentified flying objects in the strictest sense (see below for some studies).

Some scientists have argued that all UFO sightings are misidentifications of natural phenomena and historically, there was debate among some scientists about whether scientific investigation was warranted given available empirical data. Very little peer-reviewed literature has been published in which scientists have proposed, studied or supported non-prosaic explanations for UFOs. Nevertheless, over the years UFOs have been the subject of many investigations that have varied widely in scope and scientific rigor.

UFO reports became frequent after the first widely publicized U.S. sighting, reported by private pilot Kenneth Arnold in 1947, that gave rise to the popular terms "flying saucer" and "flying disc." Since then, millions of people have reported that they have seen UFOs.