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. SYN: iso- osmotic.
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is·os·mot·i·cal·ly -'mät-i-k(ə-)lē adv
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isos·mot·ic (i″soz-motґik) having the same osmotic pressure.Medical dictionary. 2011.