An unidentified flying object, or UFO, in its most general definition, is any apparent anomaly in the sky that is not identifiable as a known object or phenomenon. Such anomalies may later be identified, but depending on the evidence or lack of evidence, such an identification may not be possible, generally leaving the anomaly unexplained. While stories of unexplained apparitions have been told since antiquity, the term "UFO" was officially created in 1953 by the United States Air Force to serve as a catch-all for all such reports. It was 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." As originally defined, the term was restricted to those fraction of cases which remained unidentified after investigation, with USAF interest being for potential national security reasons and/or "technical aspects." During the late 1940s and through the 1950s, UFOs were often referred to popularly as "flying saucers" or "flying discs".