"UFO" redirects here. For other uses, see UFO (disambiguation).
Photographic image of supposed UFO, Passoria, New Jersey. 31 July, 1952
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.