Isostatic equilibrium is the balance of forces that are acting against each other in a floating solution. The concept is most often used when referring to tectonic plates in the Earth's crust.
Isostasy refers to the equilibrium that occurs between the Earth’s atmospheric layers and the tectonic plates. Due to the lithosphere and the asthenosphere, the tectonic plates float in equilibrium at different elevations in the Earth’s crust. The elevation depends on both the plates’ thickness and density. The concept of isostatic equilibrium can also explain how the Earth has a variation in topographic heights with all of the different flatlands and mountains on its surface.