double boiler

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A cooking utensil consisting of two nested pans, designed to allow slow, even cooking or heating of food in the upper pan by the action of water boiling in the lower.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun in cooking, a large pan containing hot water, into which other smaller pans are set in order to cook food at low heat (below 100C or 212F), or to keep food warm.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun two saucepans, one fitting inside the other