- (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.
The concept of hydrostatic equilibrium, frequently alluded to with non-technical approximations such as "rounded", is used in astronomy as part of the definitions of planet and dwarf planet, and sometimes of planetary object. Under planetary conditions, "solid" rock and ice are actually fluid, and will lapse into hydrostatic equilibrium if the body is massive or warm enough. Superficial deviations from perfect equilibrium, such as peaks, valleys, craters, and other irregularities of the crust which are small in scale compared to the body as a whole, are not considered significant for these definitions. (That is, the body as a whole can be considered to be in hydrostatic equilibrium even if the crust is not.)