The Nusselt number is the ratio of the amount of heat transferred across a surface in a fluid via convection to the amount of heat transferred via conduction. Both the convective and conductive heat flows are measured perpendicular to the surface.
A Nusselt number close to one indicates that the fluid has laminar flow, which means that the fluid is flowing smoothly. In contrast, a Nusselt number of much more than one indicates that the flow is turbulent. A local Nusselt number is measured at a particular point in a fluid, whereas an average Nusselt number can be calculated by integrating measurements over the whole length of the fluid surface.