third dimension

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The quality of depth or thickness in an object or a space.
• noun The quality of seeming real or lifelike.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun the quality, of depth or thickness, that differentiates a solid object from a two-dimensional image of it

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun the dimension whereby a solid object differs from a two-dimensional drawing of it