https://doi.org/10.1351/goldbook.A00381
In the representation of stereochemical relationships 'anti' means 'on opposite sides' of a reference plane, in contrast to '
syn
' which means 'on the same side', as in the following examples.
  1. Two substituents attached to atoms joined by a single bond are anti if the
    torsion angle
    (
    dihedral angle
    ) between the bonds to the substituents is greater than 90°, or
    syn
    if it is less than 90°. (A further distinction is made between
    antiperiplanar
    ,
    synperiplanar
    ,
    anticlinal
    and
    synclinal
    .)
  2. In the older literature the terms anti and
    syn
    were used to designate
    stereoisomers
    of
    oximes
    and related compounds. That usage was superseded by the terms '
    trans
    ' and 'cis' or E and Z, respectively.
  3. When the terms are used in the context of
    chemical reactions
    or
    transformations
    , they designate the relative orientation of substituents in the substrate or product:
    1. Addition to a carbon-carbon double bond:
    2. Alkene-forming
      elimination
      :
      In the examples described under (1) and (2) anti processes are always
      antarafacial
      , and
      syn
      processes are
      suprafacial
      .
    See also:
    endo, exo, syn, anti
    Sources:
    PAC, 1994, 66, 1077. (Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)) on page 1084 [Terms] [Paper]
    PAC, 1996, 68, 2193. (Basic terminology of stereochemistry (IUPAC Recommendations 1996)) on page 2199 [Terms] [Paper]