the fanciful name given to various unidentified disc- or saucer-shaped objects reported as appearing in the sky.

A disc-shaped flying craft supposedly flown or occupied by extraterrestrial beings; a UFO. Frequently as a modifier. Cf. flying saucer n.

That has not been identified; of which the identity has not been established; esp. as unidentified (flying) object: cf. UFO, n.1

An unidentified flying object; a ‘flying saucer’.