1. Mineralogic and structural changes in solid rocks caused by physical conditions which are different from those under which the rocks originally were formed.
2. A process of change in the physical structure, texture, or composition of rocks caused by agents of heat, deforming pressure, shearing stress, hot, chemically active fluids, or a combination of these, acting while the rock that is being changed remains essentially in the solid state.

Theoretically, rocks are formed when their constituents are in equilibrium with ambient physical conditions. If the conditions are changed by movements in the earth's crust or by igneous activity, metamorphism occurs to re-establish equilibrium and it changes the physical character of the rock mass.

In petrology, metamorphism accompanied by addition or removal of material: Allochemical metamorphism refers to the process of the alteration of bulk chemical composition of a rock.