Plastic deformation is the process by which a material changes shape permanently when subjected to stress beyond its elastic limit. Unlike elastic deformation, where a material returns to its original shape after the stress is removed, plastic deformation results in a new, permanent shape. This occurs in materials like metals, which can be reshaped through processes such as bending or stretching. When a metal is heated or worked on, its internal structure allows for the movement of dislocations, which are defects in the material's crystal lattice. This movement enables the material to deform without breaking, making plastic deformation essential in manufacturing processes like forging and casting.