1. Of equal tension.
2. Isosmotic.
3. Having the same concentration of solutes as the blood: an isotonic saline solution.
4. Physiology. Of or involving muscular contraction in which the muscle remains under relatively constant tension while its length changes.

[ISO- + Greek tonos, tension; see tone + -IC.]

The American Heritage® Dictionary of the English Language, Fourth Edition Copyright © 2007, 2000 by Houghton Mifflin Company. Updated in 2009. Published by Houghton Mifflin Company. All rights reserved.