An Etymological Dictionary of Astronomy and Astrophysics

1. The state of a → rigid body if, as viewed from an → inertial frame of rest: 1) the → linear acceleration of its → center of mass is zero, and 2) its → angular acceleration about any axis fixed in this reference frame is zero. The center of mass may be moving with constant velocity and the body may be rotating about a fixed axis with constant angular velocity.
   

   2. In → thermodynamics, the state of a system in which → pressure is the same every where with no other forces acting on the system except a uniform external pressure.

See also: → mechanical; → equilibrium.