equivalence class

from The Free On-line Dictionary of Computing (8 July 2008)

equivalence class

   <mathematics> 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)