Also known as water bath or water boiler. This term either refers to a baking tray half filled with water in which custards, terrines or some cakes stand to be cooked. This way the food is protected from the direct harsh heat of the oven and is cooked gently, assisted by the steam.
This term also refers to a bowl over (not touching) a pot of gently simmering water, specifically used to melt chocolate, to avoid it splitting.
It could also refer to a large container filled with hot water in which several smaller containers stand to keep their contents hot (like soups and sauces).