Some Standards attempt to reserve the term elastomer for a crosslinked material, but there is no general agreement on this. In our literature we use the terms elastomer and rubber as synonyms. Elastomers are a class of materials which differ quite obviously from all other solid materials in that they can be stretched, easily and almost completely reversibly, to high extensions. An ordinary postal rubber band illustrates this behaviour. The material is said to be elastic.
Elastomers are a special case of the wider group of materials known as polymers.
At this scale the inside of a piece of rubber can be thought of as resembling a pile of cooked spaghetti. In spaghetti, however, the chains, though intertwined, are all separate. But in most practical elastomers each chain will be joined together occasionally along its length to one or more nearby chains with just a very few chemical bridges, known as crosslinks. So the whole structure forms a coherent network which stops the chains from sliding past one another indefinitely - although leaving the long sections of chain between crosslinks free to move. The process by which crosslinks are added is known as vulcanization.