UFO stands for Unidentified Flying Object, which refers to any perceived aerial phenomenon that cannot be immediately identified or explained. UFOs can be any airborne object that does not conform to any presently known aircraft or missile type, or which cannot be positively identified as a familiar object. While technically a UFO refers to any unidentified flying object, in modern popular culture, the term UFO has generally become synonymous with alien spacecraft. However, the term UAP (Unidentified Aerial Phenomenon) is sometimes used to separate this explanation of UFOs from totally earthbound phenomena.

UFOs first became a major subject of public interest after World War II, possibly sparked by the development of rocket technology. The first widely publicized UFO sighting took place in 1947, when businessman Kenneth Arnold claimed he spotted a group of high-speed, crescent-shaped, flying objects while piloting his own plane near Mount Rainier in Washington. Sightings of strange objects in the sky became the raw materials for Hollywood to create a new genre of science fiction movies and TV shows.

While some interpret the sighting of UFOs as evidence that extraterrestrial life exists and has visited Earth, there are numerous natural phenomena that have been employed to explain UFO sightings. The U.S. government uses the phrase Unidentified Aerial Phenomena (UAPs) instead of UFOs to encourage greater scientific study and to disassociate the term from science fiction aliens.