1. Of, relating to, or characterized by having a lower osmotic pressure than a surrounding fluid under comparison.
2. A condition in which the total amount of solutes (both permeable and impermeable) in a solution is lower than that of another solution.
For instance: In kidneys, the tubular fluid is: - iso-osmotic (to plasma) when it is at the beginning of the loop of henle - hyperosmotic (to plasma) when it is at the tip of the loop - hyposmotic (to plasma) when it leaves the loop