An unidentified flying object, or UFO, is any real or apparent flying object that cannot be identified by the observer, especially those which remain unidentified after investigation. UFOs have been spotted in many different places around the world.

Reports of unusual aerial phenomena date back to ancient times , 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 UFO sighting, by private pilot Kenneth Arnold in the summer of 1947.

Many tens of thousands of UFO reports have since been made worldwide. But many sightings may yet 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 has been identified as a known object; it can be reclassified as an Identified flying object.