Coolant is the liquid — or, in some machines, the gas — that carries heat away so an engine or other machine does not get too hot. In a car it runs through the engine and into the radiator, picks up the heat, and dumps it into the air. Mechanics say “coolant”; everyday drivers often say “the water in the radiator,” though actually it is not just water. The same bottle is often called antifreeze, because it also stops the liquid freezing in winter — cooling and not-freezing are two jobs, and one mixture usually does both. You will hear the word in garages, factories, and computer labs (big computers need coolant too). It is a practical, technical word, not slang and not posh. If the coolant is low or leaking, the engine can overheat; that is why the warning light matters.