The ratio of convective to conductive heat transfer across (normal to) the boundary:
| {\rm Nu} = \frac{\rm Convective \ heat \ transfer}{\rm Conductive \ heat \ transfer} = \frac{U}{\lambda / L} =\frac{U \cdot L}{\lambda } |
where U is the convective heat transfer coefficient of the flow, L is the characteristic length, \lambda is the thermal conductivity of the fluid.
See also
Physics / Thermodynamics / Heat Transfer