pj's vs nightclothes

pj's and nightclothes both are nouns.

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

As nouns, nightclothes is a hypernym of pj's; that is, nightclothes 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
nightclothes: garments designed to be worn in bed

Other hypernyms of pj's include nightwear, sleepwear.

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