When we say that Solution A is hyperosmotic, or Solution A is isosmotic, or Solution A is hyposmotic, we compare the solution A to other solutions. These other solutions are the reference solutions: i.e. the reference points for comparison.
In Biology, the reference solutions will be either the extracellular solutions (Blood, extracellular fluids) or the intracellular solutions.

Let's say that we work on cells.

Solution A is hyperosmotic means:

- The osmolarity of Solution A is higher than that of the reference solution (The concentration of solute particles in Solution A is higher than that of the intracellular solution.)

Solution A is isosmotic means:

- The osmolarity of Solution A is the same than that of the reference solution (The concentration of solute particles in Solution A equals that of the intracellular solution.)

Solution A is hyposmotic means:

- The osmolarity of Solution A is lower than that of the reference solution (The concentration of solute particles in Solution A is lower than that of the intracellular solution.)
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