1. Property of a material to deform under load and to retain the deformation after the load is removed. 2. A material’s ability to be shaped and worked. 3. That property of plaster mortar that permits continuous and permanent deformation in any direction. 4. A plastic material is distinct from a fluid material in that it requires a measurable force (yield value) to start flow; the property exists in varying degrees in different materials and in plaster mortar is sometimes regarded as an index of working characteristics.