1. Having the same total osmotic pressure or osmolality as another fluid (ordinarily intracellular fluid); such a fluid is not isotonic if it includes solutes that freely permeate cell membranes.
The definition information for isosmotic is provided by Stedman's. You can search our medical dictionary here.
Stedman's, part of Lippincott Williams & Wilkins, provides a comprehensive line of health-science publications for healthcare professionals and medical students.