Stories of fantastical celestial apparitions have been told since antiquity, but the term UFO was officially created in 1953 by the United States Air Force to serve as a catch-all for all such reports. In its initial definition, the USAF stated that a UFOB was 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 positively identified as a familiar object. Accordingly, the term was initially restricted to that fraction of cases which remained unidentified after investigation, as the USAF was interested in potential national security reasons and/or technical aspects . (source : wikipedia)