The Collaborative International Dictionary of English v.0.48:

Banishment \Ban"ish*ment\, n. [Cf. F. bannissement.]
   The act of banishing, or the state of being banished.
   [1913 Webster]

         He secured himself by the banishment of his enemies.
                                                  --Johnson.
   [1913 Webster]

         Round the wide world in banishment we roam. --Dryden.
   [1913 Webster]

   Syn: Expatriation; ostracism; expulsion; proscription; exile;
        outlawry.
        [1913 Webster]

WordNet (r) 3.0 (2006):

banishment
    n 1: the state of being banished or ostracized (excluded from
         society by general consent); "the association should get
         rid of its elderly members--not by euthanasia, of course,
         but by Coventry" [syn: banishment, ostracism,
         Coventry]
    2: rejection by means of an act of banishing or proscribing
       someone [syn: banishment, proscription]