"coolant" in the noun sense
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"
Source: WordNet® (An amazing lexical database of English)
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