(noun) - A shape is one of the seven elements of art. When defining it within the study of art, shape is an enclosed space, the boundaries of which are defined by other elements of art (i.e.: lines, colors, values, textures, etc.).
Shapes are limited to two dimensions: length and width. Geometric shapes -- circles, rectangles, squares, triangles and so on -- have the clear edges one achieves when using tools to create them. Organic shapes have natural, less well-defined edges (think: an amoeba, or a cloud).