(1):

(n.) general equilibrium in the earth's crust, supposed to be maintained by the yielding or flow of rock material beneath the surface under gravitative stress. By the theory of isostasy each unit column of the earth, from surface to center, has approximately the same weight, and the continents stand higher than the ocean beds chiefly because the material of the crust has there less density.

(2):

(n.) The state or quality of being isostatic.