Plastic: A generic term, referring to any group of synthetic or natural materials which may be shaped when soft and then hardened, including many types of resins (amber), resinoids, polymers, cellulose derivatives, casein materials, and proteins.

Definition used with Permission, from
The New Clay : Techniques and Approaches to Jewelry Making
by Nan Roche, Chris Roche (Photographer), Sue Roche (Illustrator), Seymour Bress (Editor)
Paperback - 144 pages Reprint edition (March 1992)
Flower Valley Press, Inc.; ISBN: 0962054348