An unidentified flying object, or UFO, is any apparent flying object which cannot be identified by the observer and which remains unidentified after investigation. The term flying saucer is also sometimes used. Often a UFO is used to refer to a spacecraft of an alien life form.
Reports of unusual aerial phenomena date back to ancient times (see Ancient astronaut theories), but reports of UFO sightings started becoming 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 more sightings, however, 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.