1. A heating device consisting of a series of connected pipes, typically inside an upright metal structure, through which steam or hot water is circulated so as to radiate heat into the surrounding space.
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.

The American Heritage® Dictionary of the English Language, Fourth Edition Copyright © 2007, 2000 by Houghton Mifflin Company. Updated in 2009. Published by Houghton Mifflin Company. All rights reserved.