https://doi.org/10.1351/goldbook.S05980
A grouping within a molecular entity that may be considered a focus of
stereoisomerism
. At least one of these must be present in every
enantiomer
(though the presence of stereogenic units does not conversely require the corresponding chemical species to be
chiral
). Three basic types are recognized for molecular entities involving atoms having not more than four substituents:
  1. A grouping of atoms consisting of a
    central atom
    and distinguishable
    ligands
    , such that the interchange of any two of the substituents leads to a stereoisomer. An
    asymmetric
    atom (
    chirality centre
    ) is the traditional example of this stereogenic unit.
  2. A chain of four non-coplanar atoms (or rigid groups) in a
    stable
    conformation
    , such that an imaginary or real (restricted) rotation (with a change of sign of the
    torsion angle
    ) about the central bond leads to a stereoisomer.
  3. A grouping of atoms consisting of a double bond with substituents which give rise to
    cis-trans isomerism
    .
Source:
PAC, 1996, 68, 2193. (Basic terminology of stereochemistry (IUPAC Recommendations 1996)) on page 2219 [Terms] [Paper]