a sauna is an unnaturally hot space where one bathes or relaxes, characterized by its ability to produce löyly

necessary elements: an enclosed space, a heat source, rocks, water

unnatural heat should induce sweating and is artificially created. usually this is by means of a sauna stove but other methods, such as heating up hot rocks outside and rolling them into the hot space, can create unnaturally high heat

the hot space is where löyly is produced. usually this is a room constructed from wood paneling, timbers, or logs, although rooms of differing materials, tents, underground spaces, and other spaces can be used for sauna

löyly is the water vapor [‘steam’] created when one tosses water on rocks which have been super-heated using a sauna stove or fire

for example

‘we weren’t so keen on the lutefisk but we loved the saunas on our trip to finland’ ]
‘summer in phoenix is like a sauna’ [ but not one, because the heat is natural ]
‘the löyly in that sauna was as smooth as a well-aged whisky’
‘i like sweating in my infrared cabin but now i wish i had a sauna’

note: the sauna definition above is that of the hot space, to which most people are referring when they speak of sauna. sauna, more precisely, is composed of [ two spaces ] and, in addition to the space, the word sauna can also refer to sauna [ the activity and the verb ].