nightwear vs pyjama

nightwear and pyjama both are nouns.

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

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

nightwear: garments designed to be worn in bed
pyjama: (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, pj's.

nightwear (noun)	pyjama (noun)
garments designed to be worn in bed	(usually plural) loose-fitting nightclothes worn for sleeping or lounging; have a jacket top and trousers
	a pair of loose trousers tied by a drawstring around the waist; worn by men and women in some Asian countries