Reynolds number is defined as follows:
Re = &rho * U * D / &mu
where &rho is the specific gravity of the fluid, U  it's velocity, D
 the diamater of flow, and &mu  the viscosity.
Reynolds found that when a fluid flow is associated with a Re of
under 2100, the flow is laminar. If it is associated with a Re > 4000,
the flow is turbulent. And in between  borderline.
No clear theoretical basis for this observation, but it has been borne
out for various values of &rho, U, D, and &mu, and various fluids.
