What is meant by isostasy?

The Earth’s lithosphere, which is the crust and upper most
part of the upper mantle, is broken into tectonic plates which float in the more viscous
asthenosphere, which is the remaining part of the upper mantle (under the
lithosphere.)

Iso (same) stasis (still). Isostasy is the
process that restores isostatic equilibrium; to restore balance/buoyancy. A larger
weight (i.e. a mountain) will cause the plate to sink down. A depletion of the crust
(erosion) will cause the land (plate) to rise. As a mountaintop erodes, it will rise
(less weight), only to be eroded further, thus maintain the balance (buoyancy) of the
lithosphere upon the asthenosphere. Think of an iceberg which will sink the heavier it
is and will rise the lighter it is.  A rising mountain could be evidence of a decrease
in weight due to erosion or if the mountains are rising faster than they are eroding,
this is isostasy at work; the rising topography buoys toward equilibrium amidst the
surrounding areas/plates. If the mountain range is still Not in isostatic equilibrium,
the cause could be other factors:

Other contributing
factors to the overall isostasy gravitational and polar shift and the merging and
converging of plates.

A more simple example. A mountain
range could be the effort to fill a shallow section of the asthenosphere. Isostasy is
the Earth’s process of maintaining the balance of outer shell. The idea is that the
combination of lithosphere and asthenosphere will weigh the same everywhere on Earth:
this would be total isostatic equilibrium. When the liquid asthenosphere undergoes
density/volume shifts, lithosphere mass will make up the difference (mountains or dense
terra). When the lithosphere gains or loses weight, it will sink and displace the
underlying asthenosphere, which will then potentially cause imbalance in surrounding
areas, which must also be corrected by isostasy.