Source : Webster's Revised Unabridged Dictionary (1913)

Christendom \Chris"ten*dom\, n. [AS. cristend?m; cristen a
   Christian + -dom.]
   1. The profession of faith in Christ by baptism; hence, the
      Christian religion, or the adoption of it. [Obs.] --Shak.

   2. The name received at baptism; or, more generally, any name
      or appelation. [Obs.]

            Pretty, fond, adoptious christendoms. --Shak.

   3. That portion of the world in which Christianity prevails,
      or which is governed under Christian institutions, in
      distinction from heathen or Mohammedan lands.

            The Arian doctrine which then divided Christendom.
                                                  --Milton

            A wide and still widening Christendom. --Coleridge.

   4. The whole body of Christians. --Hooker.

Source : WordNet®

Christendom
     n : the collective body of Christians throughout the world and
         history (found predominantly in Europe and the Americas
         and Australia); "for a thousand years the Roman Catholic
         Church was the principal church of Christendom" [syn: {Christianity}]