The Collaborative International Dictionary of English v.0.48:

osmotic pressure \os*mot"ic pres"sure\, n. (Physical Chemistry)
   The pressure which a solution of a substance in a liquid
   exerts on a semipermeable membrane, through which the solvent
   can diffuse but the dissolved substance (the solute) cannot
   diffuse, when separated across the membrane from the pure
   solvent. In general, the osmotic pressure will depend almost
   proportionally up to certain concentrations upon the molal
   concentration of the solute.
   [PJC]

WordNet (r) 3.0 (2006):

osmotic pressure
    n 1: (physical chemistry) the pressure exerted by a solution
         necessary to prevent osmosis into that solution when it is
         separated from the pure solvent by a semipermeable membrane