UFO!

Unidentified flying object (commonly abbreviated as UFO) is the popular term for any aerial phenomenon whose cause cannot 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.

Studies have established that only a small percentage of reported UFOs are actual hoaxes,[5] while the majority are observations of some real but conventional object - most commonly aircraft, balloons, or astronomical objects such as meteors or bright planets - which have been misidentified by the observer as anomalies. A small percentage of reported sightings (usually 5%-20%) are classified as unidentified flying objects in the strictest sense. (see below for some studies)

UFO reports became more common after the first widely publicized US sighting - reported by private pilot Kenneth Arnold in 1947 - which gave rise to the popular terms "flying saucer" and "flying disc". Since then, millions of people have reported that they have seen UFOs.

Non-prosaic explanations of UFOs have received very little attention or support in mainstream scientific literature. Some scientists have argued that all UFO sightings are misidentifications of prosaic natural phenomena. Historically, there was debate among some prominent scientists, including members of the AAAS, regarding whether UFOs warranted serious scientific investigation.