means A device that transfers heat away from something—either cooling an engine by circulating coolant through finned tubes, or warming a room by giving off the heat of hot water or steam.

from From the Latin verb 'radiare,' meaning 'to emit beams' or 'shine out,' built on 'radius,' a ray or spoke. The English word literally names a thing that 'radiates'—throws off heat in all directions—and it arrived as a practical 19th-century coinage when industrial heating and, later, the internal-combustion engine made such heat-shedding gadgets everyday objects.