In physical sciences, "hypertonic" means that a solution exhibits greater osmotic pressure compared to a another solution, especially one separated by a semipermeable membrane. That is, the concentration of a certain solute is greater in the hypertonic solution.
Osmotic pressure is the minimum pressure necessary to counteract the force of osmosis, which is the movement of a solvent across a semipermeable membrane as a result of differences in concentration on each side. For example, if a cell is placed in a solution with a high concentration of salt, that solution is considered hypertonic, because osmosis causes water to flow into the cell to equalize the salt concentrations inside and outside of the cell. Hypertonic solutions in biology tend to cause a cell to lyse, or burst, because of excessive inward flow of water. In medicine, "hypertonic" means that something exhibits excessive tone or tension.