A water jacket is a water-filled casing surrounding a device, typically a metal sheath having intake and outlet vents to allow water to be pumped through and circulated. The flow of water to an external heating or cooling device allows precise temperature control of the device.
Water jackets are often used in watercooling. They are also used in laboratory glassware: Liebig, Graham, and Allihn condensers. Water jackets were used to cool the barrels of machine guns at the time of the First World War, but modern machine guns are air-cooled to conserve weight and hence increase portability.
Equipment such as tissue culture incubators may be enclosed in a water jacket kept at a constant temperature.