The Collaborative International Dictionary of English v.0.48:

Exorcism \Ex"or*cism\ (-s[i^]z'm), n. [L. exorcismus, Gr.
   'exorkizmo`s; cf. F. exorcisme.]
   1. The act of exorcising; the driving out of evil spirits
      from persons or places by conjuration; also, the form of
      conjuration used.
      [1913 Webster]

   2. Conjuration for raising spirits. [R.] --Shak.
      [1913 Webster]

WordNet (r) 3.0 (2006):

exorcism
    n 1: freeing from evil spirits [syn: exorcism,
         dispossession]