water jacket

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A casing containing water circulated by a pump, used around a part to be cooled, especially in water-cooled internal-combustion engines.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a container filled with water that surrounds a machine to cool it; especially that surrounding the cylinder block of an engine