Devices designed to reject excess heat through radiation into space. Typically radiators are white panels with embedded heat pipes that move heat from the components producing it to the panels where the heat is radiated away as infrared radiation. The amount of heat radiated away depends on the temperature of the radiators.
Krikkis, Rizos N., and Panagiotis Razelos. "Optimum design of spacecraft radiators with longitudinal rectangular and triangular fins." Journal of heat transfer 124, no. 5 (2002): 805-811.