Sauna is a small hot bahing chamber made of wood and heated by an oven with hot stones and humidified with water splashed onto those stones. Is is used for gentle and deep heating of all human body in order to start natural perspiration on bathers skin and further cleaning of it by various means of scrubbing and whisking.
Sauna may consist of several chambers of different temperatures: hot room, washing room, restroom and changing room. An access to outside cold water source for cooling of the body is very desirable and is usually available.
The word sauna is an ancient Finnish word referring to the traditional Finnish bath and to the bathhouse itself. Baltic countries such as Estonia share the etymology using the word saun that can also mean a hole dug out in the ground, a small cabin or cottage, such as a cabin for a fisherman.
All other nations familiar with sauna-like sweat bathing have their own traditional terms for it: saun -Estonian, pirts - Latvian, pirtis - Lithuanian, баня- Russian, мовня - Ukranian, laznia - Polish.
After WWII mostly due to a successful businesses of Finnish sauna companies (Harvia, Saunatec etc.) a term sauna gained popularity (especially in English) and was used to name almost any kind of sweat bathing in colloquial language.
In special literature sauna type of bathing usually is related to somewhat higher temperatures and low humidity levels which in reality is not always true, as microclimate in good sauna may vary depending on local tradition, sauna and sauna oven construction and bathing habits of the sauna goers.