Most users normally associate refrigeration with cold and cooling, yet the practice of refrigeration engineering deals almost entirely with the transfer of heat. This seeming paradox is one of the most fundamental concepts that must be grasped to understand the workings of a refrigeration system.
Cold is really only the absence of heat, just as darkness is the absence of light, and dryness is the absence of moisture.
Any liquid which has a temperature lower than the saturation temperature corresponding to its pressure is said to be sub cooled. Water at any temperature less than its boiling temperature (212 degrees F. at sea level) is sub cooled.