UFO vs flying saucer

UFO and flying saucer both are nouns.

Word	Noun	Adjective	Verb	Adverb
UFO	Yes	No	No	No
flying saucer	Yes	No	No	No

As nouns, UFO and flying saucer are synonyms defined as:

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

Other synonyms of UFO include unidentified flying object.

UFO (noun)	flying saucer (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