Value is the relative lightness or darkness of something. You can talk about the whole picture, objects in the picture, or the colors used in in it. Sometimes the word key is used instead.
We learn about an object’s three dimensional form by the fall of light and shadow upon it or by the value of the light upon the object.
Artists have named the different zones of light:
Chiaroscuro: Using light and dark to create the effect of a three dimensional, modeled surface.
Modeling: Another name for chiaroscuro. You may use them interchangeably.
Tenebrism: Using light as a spotlighting effect in a murky or dark scene.
Trompe l’oeil (tromp loy is close enough) This is a painting style that is so realistic that is “fools the eye.”
If the light falls realistically, and the perspective is correct, we buy it. Magic.