In the context of BDSM, a dungeon is a room or set of rooms intended and decorated for BDSM activities. Dungeons are often built and decorated to remind of the appearance of a cellar room of a medieval castle, or a torture chamber. Illumination is typically dim, with coloured light and/or candles.
A dungeon may be a private space in the home of an individual. However, most dungeons are on the premises of BDSM clubs, where they may be used for scenes with a varying numbers of participants, and for such gatherings as spanking parties. Some pro-doms (or dommes) have more or less well-equipped dungeons on their business premises, these are also called BDSM studios.
Drawing by Helga Bode.
Spanking illustration in a dungeon setting by G. Topfer.
Another illustration by G. Topfer — the setting is a convent, but the stone wall and heavy wooden door give it a dungeon feeling.