Step 1: Meaning. The word isotonic means 'equal tension (pressure)'. Solutions that exert equal osmotic pressure across a semipermeable membrane at the same temperature are isotonic.
Step 2: Condition using the formula. Osmotic pressure \(\pi = C R T\). For two isotonic solutions A and B at the same temperature, \(\pi_A = \pi_B\), and therefore \(C_A = C_B\); their solute concentrations are equal.
Step 3: Everyday example. A 0.9% NaCl solution is isotonic with blood, and a 5% glucose solution is roughly isotonic with body fluids; both are given as intravenous drips. Red blood cells kept in such solutions keep their normal shape.
Step 4: Contrast. Two solutions of different concentration are not isotonic. The more concentrated one is hypertonic and the dilute one is hypotonic; solvent moves from the hypotonic to the hypertonic side until the pressures become equal.