A term used to describe a molecule which, in a given configuration, cannot be superimposed on its mirror image. This is in contrast to achiral molecules which can be superimposed on their mirror images. The two mirror image versions of the molecule are known as "levo" (left-handed), abbreviated "L," or "dextro" (right handed), abbreviated "D," depending on which way they rotate polarized light.
From the BioTech Dictionary at http://biotech.icmb.utexas.edu/. For further information see the BioTech homenode.