Tonicity is the measure or amount of the osmotic pressure gradient.
Osmotic pressure is defined as the amount of external pressure that is required to stop the net movement of the solution across the semi permeable membrane. In terms of biology, osmosis has great importance as most of the membranes in our body are semi permeable. Permeability depends on several factors such as charge, solubility, solute size and chemistry. Osmosis is the process that provides the primary means through which water travels in and out of the cells.