What is the difference between an elastomer and a rubber?

Interesting question!

Originally, “rubber” referred to a naturally derived (organic) material. The term was first used by an English chemist (Joseph Priestley). Joseph was working with natural rubber and observed that it could “rub out” a pencil mark.

In the past, “rubber” was used to describe materials that occurred naturally in nature; and “elastomer” was used for materials that were produced synthetically ... silicone elastomers, for example.

An elastomer is a polymer that shows elastic properties. Since the terms rubber-like and elastomeric mean almost the same thing, the terms “rubber” and “elastomer” are often used interchangeably today to describe silicone elastomers.