The U.S. Air Force invented the term “unidentified flying object” in 1952 to replace the term “flying saucer.”  The Air Force defined a “UFO” as “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.” [1]
Every UFO sighting report can be placed into one of three categories:

1. Identified Flying Objects (IFOs): Those reports that are investigated and determined to be known objects.  These can include astronomical phenomena, aircraft, balloons, advertising planes, experimental aircraft, unmanned aerial vehicles, flocks of birds, and hoaxes.
2. Insufficient Data: Reports that don’t provide enough data to reach a conclusion.
3. Unknowns: These are reports by competent observers of objects which cannot be identified by the witness, and which remain unidentified after investigation by competent investigators, and whose appearance indicates that they were manufactured, and whose flight behavior indicates that they were made somewhere other than Earth. …SEE- http://alien-ufo-community.ning.com/forum/topics/are-aliens-real
4.