If the external solution balances the osmotic pressure of cytoplasm. It is said to be:

When the cell (or tissue) is placed in an isotonic solution, there is no net flow of water towards the inside or outside.  If the external solution balances the osmotic pressure of the cytoplasm it is said to be isotonic. When water flows into the cell and out of the cell and are in equilibrium, the cells are said to be flaccid.