Compression Set is defined as the decrease in thickness of a rubber which has been deformed under specific conditions of load, time and temperature. Normally shown as a percentage. Generally tested to ASTM D395 Method B. Generally a lower percentage of Compression Set indicates a better quality of Material.
A good quality Solid Rubber would have a Compression Set below 20%, most Sheet Rubber will have a Compression Set around 30%, poor quality, under cured, or badly mixed Rubber Sheeting will exhibit a Compression set of 45% or more.