The Collaborative International Dictionary of English v.0.48:

Alienism \Al"ien*ism\, n.
   1. The status or legal condition of an alien; alienage.
      [1913 Webster]

            The law was very gentle in the construction of the
            disability of alienism.               --Kent.
      [1913 Webster]

   2. The study or treatment of diseases of the mind.
      [1913 Webster]

WordNet (r) 3.0 (2006):

alienism
    n 1: an obsolete term for the study and treatment of mental
         illness
    2: the quality of being alien [syn: alienage, alienism]