UFO is an abbreviation or acronym for Unidentified Flying Object. In a literal sense it includes any airborne object whose nature is not known, but more often it is applied to those cases that are believed by some to be the spaceships of extraterrestrial aliens.
UFOs have been subject to many studies over the years, with a wide range of scope and scientific rigor. One well-known study was Project Bluebook, conducted by the United States Air Force. The idea that UFOs are in fact alien spacecraft is discounted by most scientists, because many such sightings have been explained in terms of known natural phenomena or human activity, and there is a lack of evidence corroborating eyewitness accounts which purport otherwise.
A group that is currently involved in debunking the alien spacecraft hypothesis of UFOs is the Committee for the Scientific Investigation of Claims of the Paranormal. Its main exponents are Philip Klass and James Randi.
The study of UFO claims over the years has led to interesting discoveries about atmospheric phenomena and psychology. In psychology, the study of UFOs leads to such concepts as misinterpretation, perceptual illusion, hallucination and fantasy-prone personality. The study of abduction phenomena especially shows that hypnosis is not a valid method for revealing memory.
The field of UFOs should not be confused or intermingled with "the paranormal". The UFO phenomenon need not have a paranormal explanation, though those who believe in UFOs may also have a fascination with the paranormal. UFOs are sometimes part of an elaborate UFO conspiracy theory in which the government is said to be intentionally covering up the existence of aliens. There are groups of retired government scientists who claim to have worked on captured extraterrestrial materials and technologies. These scientists are pressing for the release of the acquired technologies for the public benefit.