Tonus is the Latin equivalent of the English word tone. It is especially used to refer to:

Muscle tone, the continuous and passive partial contraction of the muscles
Arterial tone, the continuous and passive partial contraction of the arterioles
Tonicity, the ability of a solution to cause water movement
The Pythagorean interval of 9/8
A brand of fruit juice produced by the Lebedyansky division of PepsiCo in Russia