In heat transfer at a boundary (surface) within a fluid, the dimensionless Nusselt number is the ratio of convective to conductive heat transfer across (normal to) the boundary.
A Nusselt number close to one, namely convection and conduction of similar magnitude, is characteristic of laminar flow. A larger Nusselt number corresponds to more active convection, with turbulent flow typically in the range of 100 to 1000.