n.1. a. A heating device consisting of a series of pipes, typically inside an upright metal structure, through which steam or hot water circulates to heat the surrounding space by radiation or convection.
b. A similar heating device based on other technology such as the generation of heat through electrical resistance.
2. A cooling device, as in automotive engines, through which water or other fluids circulate as a coolant.
3. Physics A body that emits radiation.
4. A transmitting antenna.