What is a hypertonic solution?

When you are considering the net movement of water across a semi-permeable membrane (osmosis), you must consider the tonicity of the solutions on both sides of the membrane. Tonicity refers to the concentration of solutes (dissolved particles) in a solution. The liquid in which they are dissolved is known as the solvent. Let's consider a red blood cell in a beaker containing a solution.  

The tonicity of the solution inside of a red blood cell (intracellular solution) is about 0.9%, but to simplify, we will say 1%. If the solution that the cell is in (the extracellular solution) has a higher tonicity, we say that the solution is hypertonic to the intracellular solution. In our example, a hypertonic solution would have a solute concentration higher than 1%. Water will move more toward the hypertonic solution due to osmotic pressure so water will move from the cell, through the membrane, and into the extracellular solution.

If the tonicity of the extracellular solution is lower than that of the intracellular solution, we say that the extracellular solution is hypotonic to the intracellular solution (you could also say that the intracellular solution is hypertonic to the extracellular solution). Again, the water will move by osmosis toward the more concentrated solution so the water will move mostly into the cell and may cause the cell to burst. This happens when you add red blood cells to distilled water (0% solute).

If the tonicity of both solutions is the same, we say that the solutions are isotonic. In this case, the water will move equally into and out of the cell and the overall shape of the cell will not change.