Flying saucer (also referred to as a flying disc) is the name given to a type of unidentified flying object (UFO) with a disc- or saucer-shaped body, usually described as silver or metallic, occasionally reported as covered with running lights or surrounded with a glowing light, hovering or moving rapidly either alone or in tight formations with other similar craft, and exhibiting high maneuverability.

Although disc-shaped flying objects have been interpreted as recorded occasionally since the Middle Ages, the first highly publicized sighting by Kenneth Arnold on June 24, 1947, resulted in the creation of the term by U.S. newspapers. Although Arnold never specifically used the term "flying saucer", he was quoted at the time saying the shape of the objects he saw was like a "saucer", "disc", or "pie-plate", and several years later added he had also said "the objects moved like saucers skipping across the water." (The Arnold article has a selection of newspaper quotes.) Both the terms "flying saucer" and "flying disc" were used commonly and interchangeably in the media until the early 1950s.

Arnold's sighting was followed by thousands of similar sightings across the world. Such sightings were once very common, to such an extent that "flying saucer" was a synonym for UFO through the 1960s before it began to fall out of favor. However, the term is still often used generically for any UFO.

More recently, the flying saucer has been largely supplanted by other alleged vehicles such as the black triangle. The term UFO was, in fact, invented in 1952, to try to reflect the wider diversity of shapes being seen. However, unknown saucer-like objects are still reported, such as in the widely-publicized 2006 sighting over Chicago-O'Hare airport.

Many of the alleged flying saucer photographs of the era are now believed to be hoaxes. The flying saucer is now considered largely an icon of the 1950s and of B-movies in particular, and is a popular subject in comic science fiction.

Unidentified flying object (commonly abbreviated as UFO or U.F.O.) is the popular term for any aerial phenomenon whose cause cannot be easily or immediately identified. Both military and civilian research show that a significant majority of UFO sightings have been identified after further investigation, either explicitly or indirectly through the presence of clear and simple explanatory factors.

• Some years ago I had a conversation with a layman about flying saucers — because I am scientific I know all about flying saucers! I said "I don't think there are flying saucers'. So my antagonist said, "Is it impossible that there are flying saucers? Can you prove that it's impossible?" "No", I said, "I can't prove it's impossible. It's just very unlikely". At that he said, "You are very unscientific. If you can't prove it impossible then how can you say that it's unlikely?" But that is the way that is scientific. It is scientific only to say what is more likely and what less likely, and not to be proving all the time the possible and impossible. To define what I mean, I might have said to him, "Listen, I mean that from my knowledge of the world that I see around me, I think that it is much more likely that the reports of flying saucers are the results of the known irrational characteristics of terrestrial intelligence than of the unknown rational efforts of extra-terrestrial intelligence." It is just more likely. That is all.
  • Richard Feynman in The Character of Physical Law (1964)

• Anyway, I have to argue about flying saucers on the beach with people, you know. And I was interested in this: they keep arguing that it is possible. And that's true. It is possible. They do not appreciate that the problem is not to demonstrate whether it's possible or not but whether it's going on or not.
  • Richard Feynman in The Meaning of It All : Thoughts of a Citizen Scientist (1998)