What is a water jacket in a cars engine?

Answer 1

Most vehicle engines are cooled by water that is pumped through various passageways within the engine block, close to the combustion chambers where heat is being produced and close to other parts, such as valves and bearings, where the temperature must be controlled.

You could think of this system as a "jacket" around important parts of the engine to regulate its temperature, much as a human wears a jacket to regulate his or her temperature.

That's where the term "water jacket" comes from.