Here's a breakdown:

* Isotonic: Refers to two solutions having the same osmotic pressure. Osmotic pressure is the pressure needed to prevent the inward flow of water across a semipermeable membrane. Essentially, this means that there is no net movement of water between the two solutions.

Example: If you place a red blood cell in an isotonic solution, there will be no change in the cell's size because the concentration of water inside and outside the cell is the same.

Other terms related to concentration:

* Hypertonic: A solution with a higher concentration of solutes than another solution. Water will move out of the cell, causing it to shrink.

* Hypotonic: A solution with a lower concentration of solutes than another solution. Water will move into the cell, causing it to swell.