nightwear vs pj's

nightwear and pj's both are nouns.

Word	Noun	Adjective	Verb	Adverb
nightwear	Yes	No	No	No
pj's	Yes	No	No	No

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

nightwear: garments designed to be worn in bed
pj's: (usually plural) loose-fitting nightclothes worn for sleeping or lounging; have a jacket top and trousers

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

nightwear (noun)	pj's (noun)
garments designed to be worn in bed	(usually plural) loose-fitting nightclothes worn for sleeping or lounging; have a jacket top and trousers