An unidentified flying object (usually abbreviated to UFO or U.F.O.) is any unusual apparent object in the sky that is not identified. This term originated in the military. The UFO may be a missing-route airplane, spy airplane, enemy surprise attack or a missile, among other things, and may be a threat to national security. Therefore, an emergency take-off of battle planes is necessary. If it is not identified, it remains a UFO in the official record.Though UFO sightings have occurred throughout recorded history, modern interest in them dates from World War II (see foo fighter), and was further fueled by the Roswell UFO Incident and Kenneth Arnold's coining of the term flying saucer (both of which immediately followed the war); since when governments have investigated UFO reports, often from a military perspective- and UFO researchers have investigated, written about, and created organizations devoted to the subject. One such investigation, The UK's Project Condign report, notes that Russian, Former Soviet Republics, and Chinese authorities have made a co-ordinated effort to understand the UFO topic and that State military organizations, particularly in Russia, have done "considerably more work (than is evident from open sources)" on military applications which have stemmed from their UFO research. The report also noted that "several aircraft have been destroyed and at least four pilots have been killed 'chasing UFOs'."

Studies have established that the majority of UFOs are observations of some real but conventional object — most commonly aircraft, balloons, noctilucent clouds, nacreous clouds, 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.However, after excluding these incorrect reports, between 5% and 20% of the total remain unexplained, and so can be classified as unidentified in the strictest sense. Many such reports have been made by trained observers such as pilots, police and the military; some involve radar traces, so not all reports are visual.Proponents of the extraterrestrial hypothesis believe that these unidentified reports are of alien spacecraft, though various other hypotheses have been proposed.

While UFOs have been the subject of extensive investigation by various governments, and some scientists support the extraterrestrial hypothesis, few scientific papers about UFOs have been published in peer-reviewed journals.There has been some debate in the scientific community about whether any scientific investigation into UFO sightings is warranted.UFOs have become a relevant theme in modern culture, and the social phenomena have been the subject of academic research in sociology and psychology.The idea of UFO as alien spacecraft is an excellent and romantic theme for the fictions, being successful in literature, movie films, and television programs.