coolant

from WordNet (r) 3.0 (2006)

coolant
    n 1: a fluid agent (gas or liquid) that produces cooling;
         especially one used to cool a system by transferring heat
         away from one part to another; "he added more coolant to
         the car's radiator"; "the atomic reactor used a gas
         coolant"; "lathe operators use an emulsion of oil and water
         as a coolant for the cutting tool"

from The Collaborative International Dictionary of English v.0.48

coolant \coolant\ n.
   a fluid (gas or liquid) used to cool a device by transferring
   heat away from one part to another.
   [WordNet 1.5]