An unidentified flying object, or UFO, is any real or apparent flying object which cannot be identified by the observer and which remains unidentified after investigation. The term flying saucer is also sometimes used, although the two terms are not genuinely interchangable.



Reports of unusual aerial phenomena date back to ancient times (see Ancient astronaut theories), but modern reports and first official investigations began during World War II with sightings of so-called foo fighters by Allied airplane crews and in 1946 with widespread sightings of European ghost rockets. UFO reports became even more common after the first widely publicized United States sighting in 1947. Many tens of thousands of UFO reports have since been made worldwide. Many sightings may remain unreported due to fear of public ridicule because of the social stigma surrounding the subject of UFOs and because most nations lack any officially sanctioned authority to receive and evaluate UFO reports.



Once a UFO is identified as a known object (for example an aircraft or weather balloon), it ceases to be classified as a UFO and is reclassified as an identified object.