double boiler means: The bain-marie method is a way to cook utensils without direct heat. The utensil usually has two pans, which can be used together. Both the bottom and top pans are filled with water, while the mixture in the middle requires no direct heat. This saucepan is used most frequently to make custards and melt chocolate. You can choose from aluminum or stainless steel for your saucepans. (in Cooking Dictionary)

What else does double boiler mean?

You can cook the same way as a bain-marie in a double oven without direct heat. The saucepans can be placed on top of each other. One pan holds boiling water and one has the cooking ingredients. (in Cooking Dictionary)

Two nested saucepans that are designed for slow and even heating or cooking of food. (in Merlin Dictionary)