An instability of geostrophic (cyclostrophic) motion due to imbalance between pressure gradient and inertial forces for an infinitessimal disturbance that meridionally (radially) displaces fluid. For geostrophic motion in the Northern (Southern) Hemisphere, inertially instability may occur only if the absolute vorticity (f- partial u_g/partial y) is negative (positive). Here f is the Coriolis parameter, u_g the zonal component of geostrophic velocity, and y is distance in the meridional direction.