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.
(A) 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 90o, or syn if it is less than 90o. (A further distinction is made between antiperiplanar, synperiplanar, anticlinal and synclinal.) IUPAC ORGANIC RULES (1979); IUPAC STEREOCHEMICAL TERMINOLOGY (1993); KLYNE and PRELOG (1960).
(B) 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 That usage was superseded by the terms "trans" and "cis" or E and Z, respectively.
(1) Addition to a carbon-carbon double bond:
(2) Alkene-forming elimination:
IUPAC ORGANIC RULES (1979).
In the examples described under (1) and (2) anti processes are always antarafacial, and syn processes are suprafacial. See also IUPAC STEREOCHEMICAL TERMINOLOGY (1993) for further meaning of the term.