foreground, background, form

Foreground-the ground or parts situated, or represented as situated, in the front; the portion of a scene nearest to the viewer

Background-the plane or ground in a picture upon which all other planes or forms appear superimposed

Form-is an element of art. At its most basic, a form is a three-dimensional geometrical figure (i.e.: sphere, cube, cylinder, cone, etc.), as opposed to a shape, which is two-dimensional, or flat