It is used in medical terminology to refer to:
* Tone: The degree of tension or firmness in a muscle or other tissue.
* Sound: The quality of a sound, especially as perceived by the ear.
Examples:
* Tonometer: An instrument used to measure intraocular pressure.
* Tonic: A substance that increases muscle tone.
* Hypertonia: Increased muscle tone.
* Hypotonia: Decreased muscle tone.
* Audiotone: The sound of an auditory signal.
Keep in mind that ton/o- is not the only combining form used to refer to tone. -tonic is another common one, which usually refers to something that influences or modifies tone.