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.

Giovanni Bellini, Portrait of Doge Leonardo Loredan, 1501-05
Peter Paul Rubens, Portrait of Suzanne Fourment, 1625

Tenebrism:  Using light as a spotlighting effect in a murky or dark scene.

Francisco de Zurbaran, St. Francis Kneeling, 1635-39
Georges De La Tour, The Newborn Child, 1645
Artemisia Gentileschi, Judith with the Head of Holofernes, 1625
Michelangelo Meresi Caravaggio, Boy Being Bitten by a Lizard, 1600

Trompe  l’oeil (tromp loy is close enough) This is a painting style that is so realistic that is “fools the eye.”

Cotan, Quince, Cabbage, Melon and Cucumber, 1602

If the light falls realistically, and the perspective is correct, we buy it.   Magic.