nightdress vs nightwear

nightdress and nightwear both are nouns.

Word	Noun	Adjective	Verb	Adverb
nightdress	Yes	No	No	No
nightwear	Yes	No	No	No

As nouns, nightwear is a hypernym of nightdress; that is, nightwear is a word with a broader meaning than nightdress:

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

Other hypernyms of nightdress include intimate apparel, lingerie, nightclothes, sleepwear.

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