vapour bath vs sauna

vapour bath and sauna both are nouns.

Word	Noun	Adjective	Verb	Adverb
vapour bath	Yes	No	No	No
sauna	Yes	No	No	No

As nouns, sauna is a hyponym of vapour bath; that is, sauna is a word with a more specific, narrower meaning than vapour bath:

vapour bath: a room that can be filled with steam in which people bathe; `vapour bath' is a British term
sauna: a Finnish steam bath; steam is produced by pouring water over heated rocks

Other hyponyms of vapour bath include sweat room, Turkish bath.

vapour bath (noun)	sauna (noun)
you sweat in a steam room before getting a rubdown and cold shower	a Finnish steam bath; steam is produced by pouring water over heated rocks
a room that can be filled with steam in which people bathe; `vapour bath' is a British term