Tonicity is the osmotic pressure gradient of two solutions separated by a semi-permeable membrane. A hypotonic solution contains less solute but more water than another solution. Pure water is hypotonic compared to any solution because it does not contain any solute.
The root of the term hypotonic, "hypo," means "below." A hypotonic environment has a lower concentration of solute than the cells in the environment, and water travels by osmosis from that environment into the cells to balance the solute concentration. When the water enters a cell, the cell begins to expand. Water always moves toward a higher concentration of solute or a lower concentration of water.
It follows that these comparisons are very important in the medical field. If, for instance, a hypotonic solution is injected into a patient in which the concentration of salt is lower than the concentration of blood, the cells in contact with that solution may swell or possibly rupture. On the other hand, a solution with a higher concentration of salt than blood, called a hypertonic solution, is injected into a patient, the cells shrink. The third type of solution, an isotonic solution, does not cause swelling or shrinkage because isotonic solutions exert osmotic pressure equal to that of the blood itself.