Definitions
Properties, monographs of general purpose TPE

ISO 1382 provides the following definitions:

Elastomer: macromolecular material that returns rapidly and approximately to its initial shape and dimensions after the cessation of a low stress that has produced significant deformation.

Rubber: an elastomer which is already or can be brought to a state such that it is essentially insoluble, although swellable, in a solvent which has been brought to the boil, such as benzene, methyl ethyl ketone and ethanol-toluene azeotropes. A rubber in its modified state cannot be easily remolded by moderate heating and pressure.