nightclothes vs nightie

nightclothes and nightie both are nouns.

Word	Noun	Adjective	Verb	Adverb
nightclothes	Yes	No	No	No
nightie	Yes	No	No	No

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

nightclothes: garments designed to be worn in bed
nightie: lingerie consisting of a loose dress designed to be worn in bed by women

Other hyponyms of nightclothes include gown, night-robe, nightdress, nightgown, nightshirt, jammies, pajama, pj's, pyjama.

nightclothes (noun)	nightie (noun)
garments designed to be worn in bed	lingerie consisting of a loose dress designed to be worn in bed by women