Source : Free On-Line Dictionary of Computing

equivalence class
     
         An equivalence class is a subset whose elements
        are related to each other by an {equivalence relation}.  The
        equivalence classes of a set under some relation form a
        {partition} of that set (i.e. any two are either equal or
        {disjoint} and every element of the set is in some class).
     
        (1996-05-13)