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circulation—1. The flow or motion of a fluid in or through a given area or volume. 2. A precise measure of the average flow of fluid along a given closed curve. Mathematically, circulation is the line integral about the closed curve, where v is the fluid velocity and dr is a vector element of the curve. By Stokes's theorem, the circulation about a plane curve is equal to the total vorticity of the fluid enclosed by the curve. The given curve may be fixed in space or may be defined by moving fluid parcels. See circulation theorem.