Radiator is a set of pipes or tubes that gives off heat to its surroundings. Steam or hot-water radiators in homes transfer heat to the air in a room. When warmed, the air next to the pipes expands, becomes lighter, and rises. Cooler air from the room streams in to take its place, creating a constant circulation of air. This process is called convection, and certain types of radiators are called convectors. Radiators also heat room air by direct radiation. See Heat.