During a reaction, when the configuration of a chirality center remains unchanged.
If the atom in question is a stereo center, retention of configuration usually changes R absolute configuration into R, and S into S. When the net change in a reaction is the replacement of a ligand on a chiral center in a reactant molecule and, if in the product the replacement ligand occupies the same site on the chiral center as the replaced ligand did in the reactant. The reaction is said to occur with retention of configuration.