Stress terms obtained by transforming the equations of motion into the Reynolds equations. They are so–called in analogy to the terms in the original motion equations involving the molecular viscosity, and to further the analogy the concept of an eddy viscosity is used to perform closure on the Reynolds equations and render them soluble. The forces that give rise to the stresses are due to the fact that in a turbulent flow there are rapidly fluctuating as well as mean components. The fluctuating components oppose the mean motion and redistribute energy and other properties via a physical effect analogous to molecular friction, i.e. turbulent friction. This causes a more rapid distribution of momentum, heat and salt than would occur solely via molecular processes, and the analogous stresses are called Reynolds stresses.