Final answer:
The use of light and dark to suggest three-dimensional form in a two-dimensional artwork is called chiaroscuro, a technique developed during the Renaissance.
Step-by-step explanation:
The use of gradations of light and dark in a 2-D piece to imply a 3-D form is called chiaroscuro. This artistic technique, which was developed during the Renaissance, refers to the use of exaggerated light contrasts to create the illusion of volume on a flat surface. Not to be confused with sfumato, which is the technique of blurring or softening sharp outlines, chiaroscuro involves a more dramatic use of light and dark to suggest depth and three-dimensionality.
Note that other terms mentioned, like tenebrism, relate to styles of painting that also use strong contrasts of light and dark but are specifically associated with the dramatic illumination seen in the works of Caravaggio and his followers. Chiaroscuro, on the other hand, became an essential part of the visual vocabulary for a broad range of artists to model form and create a sense of three-dimensional space on a two-dimensional plane.