sleepwear vs nightdress

sleepwear and nightdress both are nouns.

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

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

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

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

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