Noun: The capacity of an insulating material to resist heat flow.
R-value is the measure of an insulating material’s ability to resist heat flow. The higher the R-value, the more energy savings provided by the insulation.
To test for R-value, a piece of insulation is placed between two plates in a laboratory. Heat passes through the material to test the thermal conductivity, measured in BTUs per hour. The greater the "R"-value the better the insulation capacity.