A liquid or gaseous substance designed for heat dissipation from a reactor or from a device that needs to be cooled. In the production of energy in a thermal power plant, the heat generated must be dissipated. It does not matter whether its source is fission, fusion, or biomass combustion. For this purpose, a liquid or gaseous coolant is usually used, which circulates in a closed circuit around the heat source (e.g., a reactor) and removes the heat generated. The heat is then either used directly or converted into electricity, for example in a steam turbine or thermoelectric generator. The most common coolant is water, but helium, CO₂, molten sodium, lead, or fluoride salts can also be used.

In addition to transferring heat in power generation, coolant is used to keep a variety of instruments or components at the desired temperature. Many detectors operate properly only within a certain temperature range. In thermonuclear fusion research, superconducting coils are necessary, which retain their properties only at temperatures below 4 kelvin. If very low temperatures are needed, liquid nitrogen or helium are used as coolants.