The Collaborative International Dictionary of English v.0.48:

Ouster \Oust"er\, n. [Prob. fr. the OF. infin. oster, used
   substantively. See Oust.]
   A putting out of possession; dispossession; disseizin; -- of
   a person.
   [1913 Webster]

         Ouster of the freehold is effected by abatement,
         intrusion, disseizin, discontinuance, or deforcement.
                                                  --Blackstone.
   [1913 Webster]

   2. Expulsion; ejection; as, his misbehavior caused his ouster
      from the party; -- of a person, from a place or group.
      [PJC]

   Ouster le main. [Ouster + F. la main the hand, L. manus.]
      (Law) A delivery of lands out of the hands of a guardian,
      or out of the king's hands, or a judgement given for that
      purpose. --Blackstone.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Dispossession \Dis`pos*ses"sion\, n. [Cf. F. d['e]possession.]
   1. The act of putting out of possession; the state of being
      dispossessed. --Bp. Hall.
      [1913 Webster]

   2. (Law) The putting out of possession, wrongfully or
      otherwise, of one who is in possession of a freehold, no
      matter in what title; -- called also ouster.
      [1913 Webster]

WordNet (r) 3.0 (2006):

ouster
    n 1: a person who ousts or supplants someone else [syn:
         ouster, ejector]
    2: a wrongful dispossession
    3: the act of ejecting someone or forcing them out [syn:
       ouster, ousting]