dream vs nightmare

dream and nightmare both are nouns.

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

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

Other hyponyms of dream include wet dream.

dream (noun)	nightmare (noun)
a series of mental images and emotions occurring during sleep	a terrifying or deeply upsetting dream
imaginative thoughts indulged in while awake	a situation resembling a terrifying dream
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

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