noun
an (apparently) flying object whose nature is unknown; especially those considered to have extraterrestrial origins
• Syn: ↑unidentified flying object, ↑UFO
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noun, pl ⋯ -cers [count]
: a flying object that people say they have seen in the sky, that is usually round like a saucer or disc, and that is believed by some people to be a spaceship from another world :
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a round ↑spacecraft that some people claim to have seen and that some people believe comes from another planet
Useful english dictionary. 2012.