1. n. 
The state of gravitational equilibrium between the lithosphere and the asthenosphere of the Earth such that lithospheric plates "float" at a given elevation depending on their thickness. The balance between the elevation of the lithospheric plates and the asthenosphere is achieved by the flowage of the denser asthenosphere. Various hypotheses about isostasy take into account density (Pratt hypothesis), thickness (Airy hypothesis), and pressure variations to explain topographic variations among lithospheric plates. The current model consists of several layers of different density.