From AMS Glossary
The nondimensional ratio between the product of heat advection and viscous forces and the product of heat diffusion and inertial forces in a given fluid.
It may be given as
where Cp is the specific heat at constant pressure, μ the dynamic viscosity, and k the thermal conductivity. The Prandtl number may also be defined as the ratio of the Reynolds to the Péclet numbers, or as the ratio of kinematic viscosity to thermometric conductivity.