What is an element of art that is 3-D and encloses volume?

An element of art that is three-dimensional and encloses volume is called a "form." Forms can be geometric, such as cubes and spheres, or organic, resembling natural shapes. Unlike shapes, which are flat and only have height and width, forms possess depth, allowing them to occupy space and create visual interest in a composition. Examples of form in art include sculptures and architectural structures.