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]

Moby Thesaurus II by Grady Ward, 1.0:

33 Moby Thesaurus words for "banishment":
   ban, blackball, blackballing, blacklist, boycott, boycottage,
   defrocking, degradation, demotion, depluming, deportation,
   deprivation, disbarment, disfellowship, displacement, displuming,
   exclusion, excommunication, exile, expatriation, expulsion,
   extradition, fugitation, ostracism, ostracization, outlawing,
   outlawry, proscription, relegation, rustication, stripping,
   transportation, unfrocking



Bouvier's Law Dictionary, Revised 6th Ed (1856):

BANISHMENT, crim. law. A punishment inflicted upon criminals, by compelling
them to quit a city, place, or country, for, a specified period of time, or
for life. Vide 4 Dall. 14. Deportation; Relegation.