twilight vs nightfall

twilight and nightfall both are nouns.

Word	Noun	Adjective	Verb	Adverb
twilight	Yes	Yes	No	No
nightfall	Yes	No	No	No

As nouns, twilight and nightfall are synonyms defined as:

Other synonyms of twilight include crepuscle, crepuscule, dusk, evenfall, fall, gloam, gloaming.

twilight (noun)	nightfall (noun)
the diffused light from the sky when the sun is below the horizon but its rays are refracted by the atmosphere of the earth	the time of day immediately following sunset
a condition of decline following successes
the time of day immediately following sunset

twilight (adjective)	nightfall (adjective)
lighted by or as if by twilight