coolant
n.a liquid or gas used to keep a machine or engine from getting too hot. You usually find it in cars or large industrial machines.
n.a fluid, typically liquid or gaseous, used to transfer heat away from a mechanical or industrial system to prevent overheating.
“The car engine needs more coolant before we drive.”
“The mechanic checked the radiator for leaks because the coolant level was dropping too quickly.”
“In nuclear reactors, the primary coolant must circulate constantly to absorb thermal energy from the core and prevent a catastrophic meltdown of the fuel rods.”
Uncountable when referring to the substance generally; countable when referring to specific types or brands of the fluid.
From cool + -ant.
Etymology adapted from Wiktionary, available under CC BY-SA 4.0.