pj's vs nightwear

pj's and nightwear both are nouns.

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

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

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

Other hypernyms of pj's include nightclothes, sleepwear.

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