Unidentified flying object (UFO) - observable object resembling an aircraft whose membership is not determined by earth observers.
The most complete definition of a UFO was given by the researcher of the unknown Joseph Allen Heinek: “The perception of an object or light seen in the sky or space or above the earth's surface; the phenomenon, ghost, trajectory, general dynamics and character of the glow of which does not find a logical, generally accepted explanation is not only a secret for eyewitnesses, but remains unexplained even after a close examination of all available evidence by experts who are able, if possible, to identify the phenomenon from the point of view common sense".
The hypotheses about the origin of UFOs:
The Extraterrestrial Hypothesis (ETH) - the most common hypothesis, declaring UFO like alien spacecraft. Most ufologists consider only this hypothesis;
The Natural Explanation Hypothesis or Null hypothesis - based on the fact that all unexplained cases, messages about UFOs, if they are not hoaxes, arise as a result of observation of the phenomena already known, described and studied by modern science:
Astronomical objects (bright stars, planets, meteors, re-entering man-made spacecraft, artificial satellites, and the Moon);
Aircraft (aerial advertising and other aircraft, missile launches);
Balloons (toy balloons, weather balloons, large research balloons);
Other atmospheric objects and phenomena (birds, unusual clouds, kites, flares);
Light phenomena mirages, Fata Morgana, ball lightning, moon dogs, searchlights and other ground lights, etc.
Hoaxes. Ufologists who adhere to this point of view are sometimes called "skeptics";
Psycho-social hypotheses of the origin of UFOs - it is known that UFOs were also viewed as a myth, a possible connection between the disc-shaped form of a “flying saucer” and the “mandala” - occult symbol in Hinduism and Buddhism, personifying integrity, completeness. The great interest of UFOs in the second half of the 20th century was explained by the human desire for harmony in turbulent times;
Version of anthropogenic origin UFO - the assumption that UFOs are man-made aircraft, the existence of which is hidden by the groups that own them: military, government, private corporations or the groups themselves are secret;
Version of alive UFO;
Hypotheses occurrence of UFOs in Parallel Worlds;
Hypotheses of UFO travel in time;
And other hypotheses.