Answer:
three-dimensional alternative to shape. Some also mention value, which is described as a parameter that determines the intensity of color, and pattern, which refers to repetition. As you can already imagine, these are supposed to be the basic units deployed to constitute a visual arrangement, ultimately perceived as a whole.
Thanks to the way our brain functions, we rarely interpret these units individually, unless we deliberately choose to focus on each of them in particular, or in case the artist aims to emphasize a single element in order to achieve a certain effect or to make a statement.