Are you wondering when you would normally use the terms isotonic solution and equilibrium? It is very likely that you will come across these terms when you are studying chemistry. The isotonic solution will happen when you have an equal solute concentration that is available not only inside but also outside the cell.
When you say equilibrium, this means that the solution that you are trying to study is saturated. When you have an isotonic solution, you can expect that the cells wherein it is located will remain the same. Take note that equilibrium is considered to be the state of the solution that will be located inside our outside the cell.