1.NounA room or a house designed for heat sessions.
"The hotel has a sauna in the basement."
2.NounThe act of using a sauna.
"John had a sauna after his swim.; Joanne went for a sauna after her swim."
3.NounIn some countries, a business with bath-like facilities that is actually a brothel or a place for (non-commercial) sexual encounters; a bathhouse or massage parlour.