Define isostasy ?

Isostasy is the state of equilibrium between the Earth's lithosphere (the rigid outer layer of the Earth) and the underlying asthenosphere (the semi-fluid layer beneath the lithosphere). This equilibrium is maintained by the buoyant support provided by the denser lithosphere floating on the less dense asthenosphere. Isostasy is responsible for the vertical movements of the Earth's crust in response to changes in the distribution of mass on the surface, such as the melting of glaciers or the erosion of mountains.

Isostasy is the condition of hydrostatic equilibrium within the Earth's lithosphere. It is a state in which the weight of the lithosphere, including its crust and mantle, is balanced by the buoyant force exerted by the underlying asthenosphere.