Osmosis is defined as the movement of solvent molecules from a region of their higher concentration (more dilute solution) to a region of their lower concentration (more concentrated solution), through a semipermeable membrane.

Hence, the water molecules move into a cell when a cell is placed in a hypotonic medium (resulting in an increase in cell volume) and water moves out of the cell when a cell is placed in a hypertonic medium (resulting in a decrease in cell volume).