nightmare vs incubus

nightmare and incubus both are nouns.

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

As nouns, nightmare and incubus are synonyms defined as:

nightmare (noun)	incubus (noun)
a terrifying or deeply upsetting dream	a male demon believed to lie on sleeping persons and to have sexual intercourse with sleeping women
a situation resembling a terrifying dream	someone who depresses or worries others
	a situation resembling a terrifying dream