Definition of UFO in US English:

UFO

noun

  • A mysterious object seen in the sky for which, it is claimed, no orthodox scientific explanation can be found.

    flying saucer, foo fighter
    View synonyms

Origin

1950s: acronym from unidentified flying object.

Pronunciation

UFO

/ˌyo͞o ˌef ˈō//ˌju ˌɛf ˈoʊ/