Unidentified Flying Object

UFO Abbreviation for "Unidentified Flying Object".

An Unidentified Flying Object (UFO) is any aerial phenomenon or object observed in the sky that cannot be immediately identified or explained by the observer. The term gained widespread attention after Kenneth Arnold's 1947 sighting near Mount Rainier in Washington, where he described crescent-shaped objects moving "like saucers skipping on water." A misreporting of his account led to the popularization of the term "flying saucer."

While UFOs were historically linked to speculation about extraterrestrial spacecraft, most reported sightings have been later identified as natural phenomena, human-made technologies (such as drones, weather balloons, or experimental aircraft), or optical illusions. Government investigations, including the U.S. Air Force's Project Blue Book, analyzed thousands of sightings—of 12,618 cases, 701 remained officially "unidentified."

In recent years, the term Unidentified Anomalous Phenomena (UAP) has been adopted by government and scientific bodies like NASA and the Department of Defense to reflect a broader, less culturally loaded approach to studying unexplained aerial events. As of 2026, the U.S. government has begun releasing declassified files on UAPs as part of increased transparency efforts.

The USAF defines a UFO as: Anything that relates to any airborne object which by performance, aerodynamic characteristics, or unusual features does not conform to any presently known aircraft or missile type, or which cannot be identified as a familiar object. (USAF Regulation 200-2)