A water bath is a piece of laboratory equipment used to incubate samples in water at a constant temperature over a long period of time. It is made from a container filled with heated water and is used to keep the temperature surrounding the sample consistent. Water baths can be used for a variety of applications, including enzymatic and serologic experiments. There are different types of water baths, including circulating water baths, non-circulating water baths, and shaking water baths.

In addition to laboratory use, a water bath can also refer to a cooking technique used in the kitchen. A water bath, also known as a bain-marie, is a pan of hot water placed in the oven, into which a baking dish containing the food to be cooked is placed. The hot water helps keep the temperature surrounding the food consistent, which is particularly useful for delicate bakes such as cheesecakes. Water baths can also be used to create delicate sauces, keep sauces warm, and melt chocolate or butter.

Water baths can also be used for home canning or bottling. The water bath method is used to process filled jars of food to be preserved in a large pot full of boiling water, bringing it to a rolling boil and then processing for the appropriate number of minutes required before removing from the water and allowing to cool on the bench as the jars seal. The reason for water bath canning is to heat the contents of the jars to a high enough temperature to kill any bacteria, to seal the jar to make sure nothing can get into the jars, and to ensure they can be safely stored at room temperature. Water bath canning can only be used for high acid foods as the high acid environment is inhospitable to mold, yeast, bacteria, and enzymes.

Overall, a water bath is a versatile tool used in various settings, including laboratories, kitchens, and home canning.