What is osmotic pressure?

Osmotic pressure is the pressure formed in an aqueous solution by a region of lower solute concentration upon a region of higher solute concentration forcing the passage of water from that to this more concentrated region. The strength of the osmotic pressure (in units of pressure) is equal to the pressure that is essential to apply in the solution to prevent its dilution by the entering of water by osmosis.

It is possible to apply in the solution another pressure in the contrary way to the osmotic pressure, such as the hydrostatic pressure of the liquid or the atmospheric pressure. In plant cells, for instance, the rigid cell wall makes opposite pressure against the tendency of water to enter when the cell is put under a hypotonic environment. Microscopically, the pressure contrary to the osmotic pressure does not forbid water to pass by a semipermeable membrane but it formed a compensatory flux of water in the opposite way.