A stainless steel double boiler.
double boiler definition by Webster's New World
Webster's New World College Dictionary Copyright © 2010 by Wiley Publishing, Inc., Cleveland, Ohio. Used by arrangement with John Wiley & Sons, Inc.
a pair of saucepans, one of which fits above and partly inside the other: food is cooked in the upper one by water boiling in the lower
double boiler definition by American Heritage Dictionary
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.