hydrostatic equilibrium

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A state of balance by which the internal pressure of a gaseous body, such as a star, exactly balances its gravitational pressure.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun physics The state in which a solid or liquid body has relaxed into the shape that it would have if it were a calm (equilibrium) liquid. Mathematically, this means a sphere if it is not distorted by another body and is not rotating, but an oblate or scalene ellipsoid if rotating moderately, and piriform if rotating beyond the ellipsoidal limit. When in contact with another object, such as oil drops on or in water, other shapes occur.