A bain-marie is a cooking container filled with water in which another pan or dish is placed in order to cook food more slowly or with more moisture. The term bain-marie is taken directly from the French, it literally translates as Mary’s bath. Note that when cooking with a bain-marie the pan or dish that contains the food is put directly into the hot water bath. The plural form is bains-marie. Bain-marie is often seen unhyphenated as in bain marie, but the Oxford English Dictionary only lists the hyphenated form.
A double boiler is a pan that is constructed in two parts. The lower half of a double boiler contains the boiling water, the upper half holds the food being cooked and fits above the water. The upper part of the double boiler which holds the food does not touch the water, cooking occurs because of the steam heat generated by the boiling water. The plural form is double boilers.
Examples
Did you know that bain-marie is a fancy term for a hot water bath used to control the temperature when cooking a custard or cheesecake? (The Orange County Register)
If it starts to solidify during the mixing process, scrape the mixture back into the bowl, and return to the bain marie until it liquefies again. (The Mirror)
There were heavy sauté pans, huge stockpots, fish poachers, bakeware, bains-marie, superior knives in many sizes and an array of cutting, dicing and grating tools. (The New York Times)
Place beeswax, along with shea and cocoa butter, over a double boiler, and gently warm over low heat until they melt. (The Hindustan Times)
When the caramel is set, melt half of the chocolate by using a double boiler or the microwave. (The Cherry Hill Post Courier)