Wordnet 3.0
NOUN (1)
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; 
- Example: "he added more coolant to the car's radiator"- Example: "the atomic reactor used a gas coolant"- Example: "lathe operators use an emulsion of oil and water as a coolant for the cutting tool"
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]
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"