ability of certain solids to flow or to change shape permanently when subjected to stresses of intermediate magnitude between those producing temporary deformation, or elastic behaviour, and those causing failure of the material, or rupture (see yield point). Plasticity enables a solid under the action of external forces to undergo permanent deformation without rupture. elasticity, in comparison, enables a solid to return to its original shape after the load is removed. Plastic deformation occurs in many metal-forming processes (rolling, pressing, forging) and in geologic processes (rock folding and rock flow within the earth under extremely high pressures and at elevated temperatures).

Plastic deformation is a property of ductile and malleable solids. Brittle materials, such as cast iron, cannot be plastically deformed, though at elevated temperatures some, such as glass, which is not a crystallized solid, do undergo plastic flow.

Plasticity, as the name of a science, refers either to mathematical descriptions of what happens in plastic deformation in terms of stresses, strains, and loads or to physical explanations of plastic flow in terms of atoms, crystals, grains, and motions of structural defects (dislocations) within crystals.