Balance between gravity and gas pressure. In the case of a star, gravity originates in mutual gravitational attraction of the entire mass of the star. Gas pressure is produced by nuclear reactions that heat the star's gas. ...
http://www4.nau.edu/meteorite/Meteorite/Book-GlossaryH.html
balance between gravitational downward pull on an air parcel and upward pressure gradient force arising from vertical pressure lapse. Such equilibrium applies very accurately to all but the most violently disturbed parts of the atmosphere.
http://www.advancedforecasting.com/weathereducation/weatherglossary.html
Hydrostatic equilibrium occurs when compression due to gravity is balanced by a pressure gradient which creates a pressure gradient force in the opposite direction. The balance of these two forces is known as the hydrostatic balance.
http://en.wikipedia.org/wiki/Hydrostatic equilibrium