The flow condition of a fluid system when one of the fundamental nondimensional parameters has a critical value, for example, flow of water in an open channel at a Froude number of 1, flow of gas at a Mach number of 1, flow in a pipe at Reynolds number greater than 2100, or shear flow in the atmosphere at Richardson number less than 0. 25. As each parameter crosses its critical value, the nature of the flow changes, such as from straight to wavy, from laminar to turbulent, or from linear to nonlinear.