un·i·den·ti·fied flying object (?n??-d?n?t?-f?d?)n.1. A flying or apparently flying object of an unknown nature, especially one suspected to have been sent by extraterrestrial beings.2. A flying saucer.American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.UFO (?yu??f?o?; sometimes ?yu fo?) n., pl. UFOs, UFO’s. unidentified flying object: any unexplained moving object observed in the sky, esp. one assumed to be of extraterrestrial origin. [1950?55] Random House Kernerman Webster’s College Dictionary, © 2010 K Dictionaries Ltd. Copyright 2005, 1997, 1991 by Random House, Inc. All rights reserved.