A dimensionless factor used in heat and mass transfer of fluids with turbulent flow in pipes. It is a function of Reynolds number, geometry, and boundary conditions from which the friction factor can be obtained and agrees well with convective heat transfer correlations or for determining heat transfer coefficients. It was proposed by American chemical engineer Allan P. Colburn (1904–55) and forms part of the Chilton–Colburn analogy, which is used in heat, momentum, and mass transfer.