nightwear vs night-robe

nightwear and night-robe both are nouns.

Word	Noun	Adjective	Verb	Adverb
nightwear	Yes	No	No	No
night-robe	Yes	No	No	No

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

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

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

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