(1) The potential of water molecules to move from a hypotonic solution (more water, less solutes) to a hypertonic solution (less water, more solutes) across a semi permeable membrane.
(2) A measure of the potential of water to move between regions of differing concentrations across a water-permeable membrane by using this formula: ψπ = − C R T, where ψπ is the osmotic potential, C is the concentration of solutes, R is the universal gas constant (i.e. 8.314472 J K−1 mol−1), and T is the absolute temperature.
A pure water contains no solutes, thus, it should have zero (0) water potential. And also for this reason, the value of osmotic potential of a solution is always negative since the presence of solutes will always make a solution have less water than the same volume of pure water.
In application, when two solutions are isotonic the osmotic potentials will be equal, and there will be no net movement of water molecules. When different, the solution that is hypotonic (diluted solution, less solutes more water) will have higher osmotic potential (less negative ψπ ) whereas the solution that is hypertonic (concentrated solution, more solutes less water) will have lower osmotic potential (more negative ψπ). Difference in osmotic potentials will cause water molecules to move from a hypotonic solution to a hypertonic solution.
Compare: water potential.
See also: osmosis.