R-value definition by Webster's New World
Webster's New World College Dictionary Copyright © 2010 by Wiley Publishing, Inc., Cleveland, Ohio. Used by arrangement with John Wiley & Sons, Inc.
a measurement indicating the resistance of a given thickness of material, esp. insulating material, to the flow of heat: the higher the R-value, the greater the insulation
R-value definition by American Heritage Dictionary
The American Heritage® Dictionary of the English Language, 4th edition Copyright © 2010 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
A measure of the capacity of a material, such as insulation, to impede heat flow, with increasing values indicating a greater capacity.
Origin: r(esistance) value.