a substance, usually a liquid or a gas, used to reduce the temperature of a system below a specified value by conducting away the heat produced in the operation of the system, as the liquid in an automobile cooling system or the fluid that removes heat from the core of a nuclear reactor.
a lubricant that dissipates the heat caused by friction.

1925–30; cool + -ant Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
coolant (ˈkuːlənt)
1.  a fluid used to cool a system or to transfer heat from one part of it to another
2.  a liquid, such as an emulsion of oil, water, and soft soap, used to lubricate and cool the workpiece and cutting tool during machining

Collins English Dictionary - Complete & Unabridged 10th Edition
2009 © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009
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Word Origin & History

"radiator fluid," 1930, from cool + -ant.
Online Etymology Dictionary, © 2010 Douglas Harper
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Example sentences
Its sodium coolant has a high boiling point-today's reactors use water-and
  would absorb excess heat.
The outsides of the pipes are cooled by some of the coolant circulated by the
The only long-term answer is not to add coolant or to drive more slowly, but to
  fix up the car.
The large surface area of the coiled tube coupled with the constant flow of
  fresh coolant makes for efficient condensing.
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