Answer
Osmosis is often confused with osmotic pressure. Osmotic pressure is the pressure that would have to be applied against a pure solvent to prevent the solvent molecules from passing through a semipermeable membrane into a solution by osmosis. This definition assumes that the solute molecules cannot pass through the semipermeable membrane separating solution from pure solvent. In this case, the water molecules will flow through the semipermeable membrane from high to low water concentration ( pure solvent to solution). This flow will continue until the pressure on the solution side is strong/high enough to stop the osmotic flow. The minimum pressure needed to stop the osmotic flow of the water is the osmotic pressure.
Work Step by Step
If a volume of pure solvent(water) is separated by a semi-permeable membrane from a volume of a solution(say a glucose solution), water molecules will flow from the area of higher concentration(pure solvent) into the solution, the area of lower water molecules concentration. This flow will continue until the pressure exerted from the solution side is strong enough to prevent the water molecules from passing into the solution.
Osmotic pressure is the pressure that prevents or stops the flow of water into the solution.
Formula for osmotic pressure
$\pi$(osmotic pressure)=iMRT
Where i= Van't Hoff factor
M = molar mass of solute
R = the ideal gas constant(0.0821L/K mole
T = $^{\circ}$C+273
