Unidentified Flying Object, or UFO, an object or light that cannot be readily explained as being man-made or of natural origin. UFOs usually are reported as having been seen in the air, often for only relatively brief periods. UFOs are popularly called flying saucers, a term first used in 1947 after a pilot flying near Mount Rainier, Washington, reported seeing a group of strange objects whose flight resembled that of saucers skipping on a surface of water.

Tens of thousands of UFO reports have been made since that time, and it is likely that many thousands of additional sightings have been made but not reported. Reports of UFOs have been made from all parts of the world, and often many sightings occur over a short period of time.

Although individual UFO reports vary a great deal, they often have certain common characteristics. UFOs are frequently reported as being disc-shaped, sometimes 100 feet (30 m) or more in diameter. Reports of cigar-shaped and of domed UFOs are also common. Night-time sightings are usually of strange lights, sometimes flashing and of various colors. UFOs often are reported to travel at very high speed, to make sudden stops or changes in direction, and to behave in other unusual ways. Some UFOs observed visually have also been detected by radar.

The great majority of UFO reports have proven to be of sightings of objects or natural phenomena unfamiliar to or unrecognized by the observer. Many sightings turn out to be meteors, fireballs, bright planets (in particular the planet Venus), or even the moon. Other things reported as UFOs include weather balloons, satellites, ball lightning (a rare form of lightning that appears as a slow-moving luminous sphere), and lenticular clouds (lens-shaped clouds that sometimes form over mountains). A number of reports have been revealed to be simply hoaxes. Nevertheless, a small percentage of UFO reports made by credible witnesses remain unexplained.

Of special interest are reports of so-called close encounters, sightings of UFOs at very close range. Witnesses reporting such experiences have described unusual phenomena, such as the failure of a car's electrical system in the proximity of a UFO. Some witnesses claim to have seen or made contact with occupants of UFOs.

The true nature and significance of UFOs remains largely controversial. Among the popular beliefs held concerning UFOs is that they may be spaceships from extraterrestrial civilizations. However, many scoff at this explanation because of the lack of scientific evidence for it.

Several official investigations have been made of UFOs. One of the most important was conducted by the U.S. Air Force. Beginning in 1947, it collected and studied UFO reports under a program known as Project Blue Book. In the late 1960's the Air Force requested that an independent scientific study of UFOs be made by the University of Colorado. This study, conducted under the direction of Dr. Edward Condon, concluded that UFOs did not merit scientific investigation. In 1969 the Air Force discontinued Project Blue Book, stating that those UFOs that could not be identified did not pose any threat to national security.

Several private organizations continue to investigate reports of UFOs. Among these organizations are the Amalgamated Flying Saucer Clubs of America (AFSCA), Yucca Valley, California; the Mutual UFO Network (MUFON), Seguin, Texas; the National Investigations Committee on Unidentified Flying Objects (NICUFO), Van Nuys, California; and the J. Allen Hynek Center for UFO Studies (CUFOS), Chicago, Illinois.