UFO vs unidentified flying object

UFO and unidentified flying object both are nouns.

Word	Noun	Adjective	Verb	Adverb
UFO	Yes	No	No	No
unidentified flying object	Yes	No	No	No

As nouns, UFO and unidentified flying object are synonyms defined as:

UFO and unidentified flying object: an (apparently) flying object whose nature is unknown; especially those considered to have extraterrestrial origins

Other synonyms of UFO include flying saucer.

UFO (noun)	unidentified flying object (noun)
an (apparently) flying object whose nature is unknown; especially those considered to have extraterrestrial origins	an (apparently) flying object whose nature is unknown; especially those considered to have extraterrestrial origins