Hydrostatic equilibrium

From Glossary of Meteorology

hydrostatic equilibrium

The state of a fluid with surfaces of constant pressure and constant mass (
or density) coincident and horizontal throughout.

Complete balance exists between the force of gravity and the pressure force. The relation between the pressure and the geometric height is given by the hydrostatic equation. The analysis of atmospheric stability has been developed most completely for an atmosphere in hydrostatic equilibrium.
See parcel method, slice method, quasi-hydrostatic approximation.