Definition of "ageostrophic"

It might sound puzzling that the ageostrophic geopotential (or the ageostrophic pressure in z-coordinate) has no impact on the evolution of the vorticity of a quasi-geostrophic disturbance and yet it affects the perturbation kinetic energetics (not globally but locally).

2004, Zhu Xun, Li Xiaofan, Cai Ming, Observation, Theory And Modeling Of Atmospheric Variability, page 13

Note that the ageostrophic wind has both a divergent and a nondivergent part.

2004, James R. Holton, An Introduction to Dynamic Meteorology - Volume 1, page 179

Ageostrophic circulations play a significant role in the kinematics and dynamics of a variety of synoptic- and mesoscale weather systems found within extratropical baroclinic wave regimes.

2015, Melyvn Shapiro, The Life Cycles of Extratropical Cyclones, page 239