Vorticity is a mathematical concept used in fluid dynamics. It can be related to the amount of "circulation" or "rotation" (or more strictly, the local angular rate of rotation) in a fluid.

The average vorticity in a small region of fluid flow is equal to the circulation \displaystyle{ \Gamma } around the boundary of the small region, divided by the area A of the small region.

\displaystyle{ \omega_{av} = \frac {\Gamma}{A} }