Impression materials that have mechanical properties permitting considerable elastic deformation but that return to their original form are classified as:

Elastomeric

Explanation:

An elastomer is a polymer whose main characteristic is elasticity and can even recover its shape after being deformed

The chemical composition of an elastomer is the grouping of thousands of molecules called monomers, which join together forming huge chains. These large chains of polymers are those that give elasticity since they are flexible and are intertwined in a very messy way.

When an elastomer is stretched, its molecules align, allowing many times to take a crystalline appearance. However, upon releasing the tension, it immediately returns to its original state of elastic disorder. The above distinguishes elastomers from plastic polymers.