1 Definitions (as in the book)

• Osmosis is the phenomenon in which water moves from a dilute solution to a concentrated solution through a semi-permeable membrane that separates the two solutions. (A semi-permeable membrane lets the small solvent molecules (water) pass but blocks the larger or hydrated solute particles; the plasma membrane of a cell behaves like this.)
• Osmotic pressure is the pressure that arises in a solution due to the difference in the concentration of solutes between its parts. It makes water move from the solution of lower concentration (lower osmotic pressure) toward the solution of higher concentration (higher osmotic pressure).

Notice the apparent opposite of "pressure pushes out": a more concentrated solution has a higher osmotic pressure, which means a stronger tendency to draw water in.