From AMS Glossary
A similarity relationship between surface flux of a variable and the mean vertical profile of that variable.
These are generically of the form
where M is mean wind speed, u* is the friction velocity (a term representing the momentum flux), k is von Kármán's constant, z is height above ground, d is displacement distance, z0 is roughness length, ψ is an empirical stability correction term, and L is the Obukhov length.
Stull, R. B. 1988. An Introduction to Boundary Layer Meteorology. 666 pp.