Definition of "vorticity"

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

This results from the vorticity component of shear, whereas elongated cells would further be affected by the rate of strain component.

2009 February 20, William M. Durham et al., “Disruption of Vertical Motility by Shear Triggers Formation of Thin Phytoplankton Layers”, in Science‎, volume 323, number 5917, pages 1067–1070

An extreme version of vorticity is a vortex. The vortex is a spinning, cyclonic mass of fluid, which can be observed in the rotation of water going down a drain, as well as in smoke rings, tornados and hurricanes.

2013 March, Frank Fish, George Lauder, “Not Just Going with the Flow”, in American Scientist‎, volume 101, number 2, archived from the original on 1 May 2013, page 114