nightmare vs dream

nightmare and dream both are nouns.

Word	Noun	Adjective	Verb	Adverb
nightmare	Yes	No	No	No
dream	Yes	No	Yes	No

As nouns, dream is a hypernym of nightmare; that is, dream is a word with a broader meaning than nightmare:

Other hypernyms of nightmare include dreaming.

nightmare (noun)	dream (noun)
a terrifying or deeply upsetting dream	a series of mental images and emotions occurring during sleep
a situation resembling a terrifying dream	imaginative thoughts indulged in while awake
	a fantastic but vain hope (from fantasies induced by the opium pipe)
	a state of mind characterized by abstraction and release from reality
	a cherished desire
	someone or something wonderful

nightmare (verb)	dream (verb)
	have a daydream; indulge in a fantasy
	experience while sleeping