Isotonic solutions are solutions having the same

Two solutions that have the same osmotic pressure across a semipermeable membrane are referred to as isotonic solutions. It has the same osmolarity (solute concentration), as the other solution. 
If the medium has exactly the same water concentration as the cell, there will be no movement of water across the membrane. Hence, such a solution is known as an isotonic solution.
A solution is isotonic when its mole concentration is the same as another solution.