Isotonic solutions.

Explanation

### Step-by-Step Solution for Defining Isotonic Solutions

**Step 1: Understanding Osmotic Pressure**
- Osmotic pressure is a colligative property of solutions, which is the pressure required to prevent the flow of solvent into a solution through a semipermeable membrane. It is directly proportional to the concentration of solute particles in the solution.

**Step 2: Definition of Isotonic Solutions**
- An isotonic solution is defined as a solution that has the same osmotic pressure as another solution. This means that when two solutions are separated by a semipermeable membrane, there will be no net movement of solvent molecules between them.

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