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. Both military and civilian research show that a significant majority of UFO sightings have been identified after further investigation, either explicitly or indirectly through the presence of clear and simple explanatory factors (see Occam's Razor). 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.

Studies have established that only a small percentage of reported UFOs are actual hoaxes, while the majority 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. A small percentage of reported sightings (usually 5 %–20 %) are classified as unidentified flying objects in the strictest sense (see below for some studies). Certain scientists have argued that all UFO sightings, in the strictest sense, are misidentifications of prosaic 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. UFO reports became frequent after the first widely publicized US 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.

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