From AMS Glossary
(Also called geostrophic deviation, ageostrophic wind.) The vector (or sometimes only the magnitude) difference between the real (or observed) wind and the geostrophic wind.
In the atmospheric boundary layer, winds can be subgeostrophic due to turbulent drag against the surface. This causes a steady-state geostrophic departure, written here as separate Cartesian components (V - Vg) and (U - Ug), or
where (U, V) are horizontal wind components, CD is a drag coefficient, M is total wind speed, (Ug, Vg) are geostrophic winds components, fc is the Coriolis parameter, and zi is depth of the boundary layer. The equations above are only approximate (they assume a slab boundary layer with no entrainment), but illustrate the effect.