double boiler

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. 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.

Wiktionary

1. n. 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.

WordNet 3.0

1. n. two saucepans, one fitting inside the other