Stereochemical configuration refers to the arrangement of atoms in a molecule in three-dimensional space, especially with regard to the differences between enantiomers (a pair of chiral isomers that are direct, non-superimposable mirror images of each other). The arrangements are specified in chemical formulas with the letters R, S, L, and/or D.
In the R and S system, each asymmetric carbon atom is classified as R (for rectus, Latin for right) if the chemical groups attached to it have decreasing priorities in a clockwise direction. Alternately, they are classified as S (for sinister, Latin for left) if the chemical groups attached to it have decreasing priorities in a counterclockwise direction, when the carbon atom in question is viewed along the bond of the chemical group with the lowest priority. Priorities are assigned to chemical groups according to the Cahn-Ingold Prelog priority rules (generally, the larger the group the higher the priority).
L and D designations are given if the enantiomers have optical activity; that is, if they will rotate polarized light. The member of the pair which rotates polarized light clockwise is dextrorotatory, or D, and the member of the pair which rotates polarized light counterclockwise is levorotatory, or L (there is always one of each in every pair). This is also often referred to as handedness, where D is righthanded and L is lefthanded.
From the BioTech Dictionary at http://biotech.icmb.utexas.edu/. Used with permission. For further information see the BioTech homenode.