The pressure of solution that just prevents the flow of solvent through a semipermeable membrane is called Osmotic Pressure.
When a solution and pure solvent are separated by a semipermeable membrane, the solvent naturally flows from the region of lower solute concentration (pure solvent) to the region of higher solute concentration (solution). This process is known as osmosis.
If an external pressure is applied on the solution side and gradually increased, a point is reached where the flow of solvent stops completely. The minimum pressure required to stop this osmotic flow is called osmotic pressure.
Thus, the pressure that just prevents the flow of solvent is known as Osmotic Pressure.