flying saucer vs fantasm

flying saucer and fantasm both are nouns.

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

As nouns, fantasm is a hypernym of flying saucer; that is, fantasm is a word with a broader meaning than flying saucer:

flying saucer: an (apparently) flying object whose nature is unknown; especially those considered to have extraterrestrial origins
fantasm: something existing in perception only

Other hypernyms of flying saucer include apparition, phantasm, phantasma, phantom, shadow.

flying saucer (noun)	fantasm (noun)
an (apparently) flying object whose nature is unknown; especially those considered to have extraterrestrial origins	something existing in perception only
	a ghostly appearing figure