nightwear vs nightgown

nightwear and nightgown both are nouns.

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

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

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

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

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