Popular term for Unidentified Flying Objects or UFOs. The terms flying saucer and UFO were used somewhat synonymously in the 1950s, but the former term was eventually adopted by those who claimed to have positive evidence that what others called UFOs were in fact extraterrestrial crafts. The term originated from a report by Kenneth Arnold, whose 1947 sightings of UFOs near Mt. Rainier in Washington began the modern UFO era.
Clark, Jerome. The Emergence of a Phenomenon: UFOs from the Beginning through 1959. Vol. 2 of The UFO Encyclopedia. Detroit: Omnigraphics, 1992.