UFO vs apparition

UFO and apparition both are nouns.

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

As nouns, apparition is a hypernym of UFO; that is, apparition 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
apparition: something existing in perception only

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

UFO (noun)	apparition (noun)
an (apparently) flying object whose nature is unknown; especially those considered to have extraterrestrial origins	an act of appearing or becoming visible unexpectedly
	something existing in perception only
	the appearance of a ghostlike figure
	a ghostly appearing figure