Pronunciation: (yOO'ef'ō' or, sometimes, yOO'fō), [key]
—pl. UFO's, UFOs.
any unexplained moving object observed in the sky, esp. one assumed by some observers to be of extraterrestrial origin. Also called unidentified flying object. Cf. flying saucer.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.