flying saucer vs phantasm

flying saucer and phantasm both are nouns.

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

As nouns, phantasm is a hypernym of flying saucer; that is, phantasm 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
phantasm: something existing in perception only

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

flying saucer (noun)	phantasm (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