1. Capable of being molded; pliant; supple. 2. Any of a number of synthetic polymeric substances that can be given any required shape. 3. In stress analysis, refers to stress/strain behavior beyond the elastic range; plastic deformation usually implies some permanent shape change. 4. A condition of freshly mixed concrete, mortar, or cement paste indicating that it is workable and readily remoldable, is cohesive and has an ample content of fines and cement but is not over wet.