Form: the appearance of having three-dimensions (3-D), whether the artwork is two-dimensional or three-dimensional.
Three-dimensional (3-D) objects have height, width, and depth. Form deals mostly with three-dimensional art, but also applies to two-dimensional art in the respect that we can make objects look like they have form by using value (shading - a gradual change in value). This change is used to create the illusion of three-dimensional form.
To illustrate three-dimensionality, look at the images* below (click on the image to enlarge it):