Unidentified flying object (commonly abbreviated UFO ) is a term used to refer to any aerial phenomenon whose cause can not be easily or immediately determined. Both military and civilian research show that a significant majority of UFO sightings are identified after further investigation, either explicitly or indirectly (through the presence of clear and simple explanatory factors: see Occam's Razor .  The United States Air Force , who coined the term in 1952, initially defined UFOs as those objects which remain unidentified after scrutiny by expert investigators,  though the term UFO is often used more generally to describe any sighting unidentifiable to the reporting observer(s). Popular culture frequently takes the term UFO as a synonym for alien spacecraft . Some investigators now prefer to use the broader term Unidentified Aerial Phenomenon (or UAP ), to avoid the confusion and speculative associations that have become attached to UFO .