An unidentified flying object (usually abbreviated to UFO or U.F.O.) is any unusual apparent object or phenomenon in the sky whose cause cannot be identified by the observer, or (in a narrower definition) by investigators; though in popular usage it more loosely means alien spacecraft, being one explanation (among several) offered for such sightings. Though UFO sightings have occurred throughout history, modern interest in them dates from World War II, 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.
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 so-called 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.
