nightie vs sleepwear

nightie and sleepwear both are nouns.

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

As nouns, sleepwear is a hypernym of nightie; that is, sleepwear is a word with a broader meaning than nightie:

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

Other hypernyms of nightie include intimate apparel, lingerie, nightclothes, nightwear.

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