The condition, of a moving, inviscid fluid, in which the horizontal force per unit area exactly balances the horizontal pressure at all points of the field in question: 2 Ω x V = δ H p ρ , in which Ω is the angular velocity of the Earth, V the geostrophic velocity of the fluid, p the pressure, ρ the specific gravity and δ H the horizontal del operator. In geodetic applications, dynamic equilibrium results when the difference in hydro-static pressure between two points is balanced by the Coriolis force per unit area.