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.
An omen is a phenomenon that is believed to foretell the future, often signifying the advent of change.
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Culturally, UFOs are associated with claims of visitation by extraterrestrial life or government-related conspiracy theories, and have become popular subjects in fiction.
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UFOs are often identified after their sighting.
Sometimes, however, UFOs cannot be identified because of the low quality of evidence related to their sightings.
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.
The United States Air Force is the aerial warfare service branch of the United States Armed Forces and one of the seven American uniformed services.
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."
In modern usage, a missile is a self-propelled precision-guided munition system, as opposed to an unguided self-propelled munition, referred to as a rocket.
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".
During the late 1940s and through the 1950s, UFOs were often referred to popularly as "flying saucers" or "flying discs".
A flying saucer is a descriptive term for a supposed type of flying craft having a disc or saucer-shaped body, commonly used generically to refer to an anomalous flying object.
The term UFO became more widespread during the 1950s, at first in technical literature, but later in popular use.
UFOs garnered considerable interest during the Cold War, an era associated with a heightened concern for national security.
The Cold War was a state of political and military tension after World War II between powers in the Western Bloc and powers in the Eastern Bloc.
Various studies have concluded that the phenomenon does not represent a threat to national security nor does it contain anything worthy of scientific pursuit.