coolant

noun

1. A medium, usually fluid, used to draw heat from an object.

   “Coolant is used in car engines and industrial processes, where excess heat could cause machine damage.”

2. A fluid agent (gas or liquid) that produces cooling; especially one used to cool a system by transferring heat away from one part to another

   “he added more coolant to the car's radiator”

   “the atomic reactor used a gas coolant”

   “lathe operators use an emulsion of oil and water as a coolant for the cutting tool”