Perspective - providing a sense of depth (usually thought of in terms of foreground, middle ground and background). Methods for providing perspective include using a horizon line, relative size and scale of objects, linear perspective (converging lines to convey distance), and color and value (darker, richer colors appear to be in the foreground).
I've always admired people with a good perspective for drawings and teaching in this way. This is not just limited to design, even though it is more related to design, it is also a wawy of communication, making depth, context, and color and value, making it tone, and also value.