from The American Heritage® Dictionary of the English Language, 4th Edition
- n. A flying or apparently flying object of an unknown nature, especially one suspected to have been sent by extraterrestrial beings.
- n. A flying saucer.
from Wiktionary, Creative Commons Attribution/Share-Alike License
- n. Anything not readily explainable appearing to move through or be suspended in the air, primarily used to refer to objects that seem to be at least of small familiar aircraft size. Abbreviated UFO.
- n. An alien spacecraft. More commonly called a UFO.
from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.
- n. an (apparently) flying object whose nature is unknown; especially those considered to have extraterrestrial origins
The phrase is usually attributed to Edward J. Ruppelt who coined it as a replacement of the terms flying saucer and flying disk. (Wiktionary)