Osmotic balance occurs when the water vapor pressure of the intracellular fluid equals the vapor pressure of the extracellular fluid.

If a solution bound by a semipermeable membrane is placed within surroundings of pure water, the vapor pressure of the pure water will be greater than that of the bound solution.

The hydrostatic pressure necessary to compensate and achieve balance (and stop the flow of water) is called the osmotic pressure of the solution.