What is "compression set"

n. The tendency of soft materials, such as elastomers, to undergo permanent deformation; the tendency of such materials to have a memory or to lose their ability to recover in a completely elastic manner.

The compression set ( ASTM D395) of a material is the permanent deformation remaining when a force (that was applied to it) is removed. The term is normally applied to soft materials such as elastomers. There are two ways this is normally measured; compression set A and compression set B.