A rubber like material that when compressed and released will return to 90% of its original shape in less than five seconds.
If it takes more than five seconds, or if the part returns to less than 90% of its original shape we say the material is a plastic.
Viton® is an elastomer. Teflon® is a plastic.
This is the reason O-rings are made 10% oversize. If you measure a 1/8 inch cross section O-ring it does not measure 0.125 inches, it measures 0.139 inches instead, even though the tolerance is plus or minus 0.003 inches
® E.I. Dupont