The French call this cooking technique bain marie. It consists of placing a container (pan, bowl, soufflé dish, etc.) of food in a large, shallow pan of warm water, which surrounds the food with gentle heat. The food may be cooked in this manner either in an oven or on top of a range. This technique is designed to cook delicate dishes such as custards, sauces and savory mousses without breaking or curdling them. It can also be used to keep cooked foods warm.
From The Food Lover's Companion, Fourth edition by Sharon Tyler Herbst and Ron Herbst. Copyright © 2007, 2001, 1995, 1990 by Barron's Educational Series, Inc.