Excess pressure required to maintain osmotic equilibrium between a solution and the pure solvent separated by a membrane permeable only to the solvent: \[\mathit{\Pi} = -\frac{R\ T}{V_{\text{A}}}\ \ln a_{\text{A}}\] where V A, aA are the partial molar volume and activity of solvent A for an incompressible fluid. For ideal dilute solutions, Π = c B R T = ρ B R T M B, where entities B are individually moving solute molecules, ions, etc., regardless of their nature, cB, ρ B are the amount and mass concentration of the solutes, and M B is the mass average molar mass of the solutes. The amount is sometimes expressed in osmol (meaning a mole of osmotically active entities), but this usage and the corresponding term osmolarity are discouraged.