double boiler; jacketed kettle
A cooking pot with two layers, where the outer jacket holds water or steam to heat the inner pot gently. Used for melting chocolate, making custards, or in commercial kitchens.
When melting chocolate, using a double boiler makes it less likely to burn.
They make large batches of soup in a commercial jacketed kettle.
Compound of 二重 (にじゅう, 'double') and 釜 (かま, 'pot, kettle'). The term directly describes the double-layered construction.