To begin with, osmotic pressure is the minimum pressure necessary to prevent a dilute solution from moving through a semipermeable membrane to a concentrated solution. For more details, see this article.
In their natural state, dilute solutions move by osmosis into concentrated solutions. In this situation, the diluted solution is said to be hypotonic while the concentrated solution is said to be hypertonic. An equivalent solution, therefore with the same osmotic pressure, is called isotonic.
Changes in Osmotic Pressure
Typically, fluctuations in osmotic pressure are caused by the composition of the solutions. This means that water with a higher concentration of dissolved or suspended solids will have a higher osmotic pressure. Conversely, the purer the solution, such as ultrapure water, the lower the osmotic pressure.
Hypertonic solution or Hypertonicity
A solution is defined as hypertonic when it has a lower osmotic pressure than the reference solution.
Hypotonic solution or Hypotonicity
A hypotonic solution is one with a higher osmotic pressure.
Isotonic solution or Isotonicity
Finally, isotonic solutions have the same concentrations as the reference solutions. This means that they have the same osmotic pressure.