A current in which an approximate geostrophic balance between Coriolis and pressure gradient forces holds, but for which other terms, generally the inertial terms involving temporal change or advective acceleration, play a key dynamic role (through vortex stretching effects) despite their relatively small magnitude. For the geostrophic approximation to hold, the flow must be nearly steady (timescale >> pendulum day), weak, and large-scale (small Rossby number), with negligible friction (small Ekman number).