bilateral symmetry

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

• noun Symmetrical arrangement, as of an organism or a body part, along a central axis, so that the body is divided into equivalent right and left halves by only one plane.

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

• noun the property of being symmetrical about a vertical plane
• noun biology the characteristic, in animals, of being symmetric about a plane running from head to tail

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

• noun the property of being symmetrical about a vertical plane