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Wiktionary
unidentified flying object

n. 1 Anything not readily explainable appearing to move through or be suspended in the air, primarily used to refer to objects that seem to be at least of small familiar aircraft size. Abbreviated UFO. 2 (context colloquial English) An alien spacecraft. More commonly called a UFO.

WordNet
unidentified flying object

n. an (apparently) flying object whose nature is unknown; especially those considered to have extraterrestrial origins [syn: UFO, flying saucer]

Wikipedia
Unidentified flying object

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. Culturally, UFOs are associated with claims of visitation by extraterrestrial life or government-related conspiracy theories, and have become popular subjects in fiction. While UFOs are often later identified, sometimes identification may not be possible owing to the usually low quality of evidence related to UFO sightings (generally anecdotal evidence and eyewitness accounts).

Stories of fantastical celestial apparitions have been told since antiquity, but the term "UFO" (or "UFOB") was officially created in 1953 by the United States Air Force (USAF) 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" (see Air Force Regulation 200-2).

During the late 1940s and through the 1950s, UFOs were often referred to popularly as " flying saucers" or "flying discs". 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. Various studies have concluded that the phenomenon does not represent a threat to national security nor does it contain anything worthy of scientific pursuit (e.g., 1951 Flying Saucer Working Party, 1953 CIA Robertson Panel, USAF Project Blue Book, Condon Committee).

Usage examples of "unidentified flying object".

If everything goes well, we can talk about letting one of you get on the ground with me next trip though durned if I don't think a stacked redhead is more conspicuous than an unidentified flying object.

Witnesses who were unlucky enough to have a close encounter with an unidentified flying object, usually a dazzlingly brilliant aerial light, are exposed to actinic rays .

People who reported seeing things got laughed at, mostly because they call any unidentified flying object a flying saucer.

The simplest explanation was that they were related to ball-lightning, responsible for so many 'Unidentified Flying Object' reports on Earth and Mars.

A rush of flying-saucer sightings followed inevitably--alleged to be landing-parties from M-387--and in Peoria, Illinois, a picnicking party sighted an unidentified flying object shaped like a soup spoon, the handle obviously being its tail.