coolant

noun

1. 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

   noun.substance

   Examples

   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.

   Source: WordNet 3.0