In
continuum mechanics, a
fluid is said to be in
hydrostatic equilibrium or
hydrostatic balance when it is at rest, or when the flow velocity at each point is constant over time. This occurs when external forces such as
gravity are balanced by a
pressure gradient force. For instance, the pressure-gradient force prevents gravity from collapsing
Earth's atmosphere into a thin, dense shell, whereas gravity prevents the pressure gradient force from diffusing the atmosphere into space.