fantasy vs pipe dream

fantasy and pipe dream both are nouns.

Word	Noun	Adjective	Verb	Adverb
fantasy	Yes	No	Yes	No
pipe dream	Yes	No	No	No

As nouns, pipe dream is a hyponym of fantasy; that is, pipe dream is a word with a more specific, narrower meaning than fantasy:

fantasy: imagination unrestricted by reality
pipe dream: a fantastic but vain hope (from fantasies induced by the opium pipe)

Other hyponyms of fantasy include dream, fantasy life, phantasy life, fairyland, fantasy world, phantasy world.

fantasy (noun)	pipe dream (noun)
imagination unrestricted by reality	a fantastic but vain hope (from fantasies induced by the opium pipe)
something many people believe that is false
fiction with a large amount of imagination in it

fantasy (verb)	pipe dream (verb)
indulge in fantasies