UFO vs fantasm

UFO and fantasm both are nouns.

Word	Noun	Adjective	Verb	Adverb
UFO	Yes	No	No	No
fantasm	Yes	No	No	No

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

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

Other hypernyms of UFO include apparition, phantasm, phantasma, phantom, shadow.

UFO (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