stereoisomerism

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

• noun Isomerism created by differences in the spatial arrangement of atoms in a molecule.

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

• noun chemistry A form of isomerism in which atoms are arranged differently about a chiral centre (or centre of asymmetry); they exhibit optical activity; in a molecule with a single chiral centre the two isomers (enantiomers) are mirror images of each other; in a molecule with multiple chiral centres the isomers (diastereoisomers) are not normally mirror images.