A UFO, or unidentified flying object, is technically any object or phenomenon observed in the sky that cannot be readily identified. In popular usage, it has come to refer mainly to flying objects or lights believed to be alien spacecraft by observers, especially those that fit the general description of a "flying saucer". Originally called "foo fighters" or "ghost rockets" when spotted by airmen in World War II, UFOs during the '50s, spurred on by the Roswell incident in 1947 and later amplified by Cold War tensions. UFO sightings have been interpreted by many as alien spaceships or secret military vehicles, while others see them as optical illusions, meteorological events, or simply misidentified aircraft and birds.
In video games "UFO" is somewhat of a misnomer, since they are most often depicted as full-on alien ships, sometimes even controlled by the player. Truer to life are UFOs that are hidden in games as easter eggs, their nature unexplained; Grand Theft Auto V is a recent example of this.