Tone Definition

tone: 

Google: A way of showing greater firmness or strength within an image. With a Range in-between, (this range is called) Gamut. 

My own: The way the light falls on a 3D object. There are highlights and shadows creating this tone. Without tone, Form doesn't exist.

For example, within this image you can see the light is coming towards the left hand side of the face so this side is HIGHLIGHTED the nose area is the TRANSITIONAL area and the other side of the face is CORE SHADOW tone. 

Tone examples: 
Nature,
Clouds,
Trees,
Grass,
Flowers,
Skin,
Human faces,
Objects.