The Free On-line Dictionary of Computing (30 December 2018):

equivalence relation

    A relation R on a set including elements a, b,
   c, which is reflexive (a R a), symmetric (a R b => b R a) and
   transitive (a R b R c => a R c).  An equivalence relation
   defines an equivalence class.

   See also partial equivalence relation.

   (1996-05-13)