A theorem useful for ideal flows that have negligible viscous forces and a nearly constant density (incompressible flows). Such flows are potential flows and are applicable to the aerodynamics of wings. One characteristic of potential flows is a lack of vorticity. Vorticity characterises the solid-body-like rotation of the fluid particles as they translate and deform. If a small element of fluid in a flow were instantaneously frozen, vorticity would indicate its spinning motion. Potential flows have zero vorticity.