Source : Free On-Line Dictionary of Computing

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)