“The compounds that have the same constitution and sequence of covalent bonds but differ in relative positions of their atoms or groups in space are called stereoisomers.”
The Cahn-Ingold-Prelog system designs a rule that is used to define the stereochemical configuration.
(from the Latin rectus, which means right-handed) or (from the Latin sinister, which means left-handed).
R isomer has its relative direction of the priority order in a clockwise direction while S is in an anticlockwise direction.
An example of alanine in R/S configuration is shown below:
