UFO vs phantom

UFO and phantom both are nouns.

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

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

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

UFO (noun)	phantom (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

UFO (adjective)	phantom (adjective)
	something apparently sensed but having no physical reality