any object that radiates energy
"The glowing ember acted as a radiator, emitting intense heat into the cold cave."
heater consisting of a series of pipes for circulating steam or hot water to heat rooms or buildings
"The old radiator in the hallway began hissing as it circulated hot water to warm up the cold room."
a mechanism consisting of a metal honeycomb through which hot fluids circulate; heat is transferred from the fluid through the honeycomb to the airstream that is created either by the motion of the vehicle or by a fan
"The car's radiator, a metal honeycomb through which hot coolant flows, cools the engine by transferring heat to the air pushed past it by the vehicle's forward motion."
Anything which radiates or emits rays.
"The scientist warned that even a small black object could act as an effective radiator in the vacuum of space by emitting thermal energy."
In plain English: A radiator is a metal box that heats up air to make your room warmer.
"The old radiator in the hallway hissed loudly before finally turning off."
The word radiator comes from combining the Latin root radiare, meaning to emit rays or heat, with the suffix -or. It entered English as a doublet of "radiatori," reflecting its original function of emitting warmth through radiation.