In fluid dynamics the Milne-Thomson circle theorem or the circle theorem is a statement giving a new stream function for a fluid flow when a cylinder is placed into that flow. It was named after the English mathematician L. M. Milne-Thomson.
Let  be the complex potential for a fluid flow, where all singularities of  lie in . If a circle  is placed into that flow, the complex potential for the new flow is given by
with same singularities as  in  and   is a streamline. On the circle , , therefore