Immersion cooking is done using a device that sits in a tub or pot of water and heats it to a specific temperature, usually for a specific length of time.

Also called sous-vide, French for "under (in a) vacuum (environment)," one typically seals the food and sucks out all the air to remove the air and to keep the bad from floating in the water (so that the heated water is brought as close to bear on the food being cooked as possible.

This method of cooking offers unparalleled control over the final temperature of the food. In the case of cuts of meat especially it replaces the skill needed to unit multiple variables such as internal temperature and doneness with oven or skillet temperature and time in oven or skillet with the depth of sear (or maillard reaction) and many other factors. It almost completely removes the risk involved. When the item being cooked is an incredibly expensive piece of meat, the risk of destroying it or at least not getting it just right is largely palliated.

For extended-time cooking (many hours), it's a good idea to cover and/or wrap the bath in kitchen towels to insulate it.