a combining form with the meanings “stretching,” “tension,” “tone,” used in the formation of compound words: tonometer.
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019
Medicine definitions for tono-
Tone; tension; pressure:tonography.
The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.