| Noun | 1. | coolant - 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" agent - a substance that exerts some force or effect fluid - a substance that is fluid at room temperature and pressure |