The Collaborative International Dictionary of English v.0.48:

Excommunication \Ex`com*mu`ni*ca"tion\, n. [L. excommunicatio:
   cf. F. excommunication.]
   The act of communicating or ejecting; esp., an ecclesiastical
   censure whereby the person against whom it is pronounced is,
   for the time, cast out of the communication of the church;
   exclusion from fellowship in things spiritual.
   [1913 Webster]

   Note: excommunication is of two kinds, the lesser and the
         greater; the lesser excommunication is a separation or
         suspension from partaking of the Eucharist; the greater
         is an absolute execution of the offender from the
         church and all its rights and advantages, even from
         social intercourse with the faithful.
         [1913 Webster]

WordNet (r) 3.0 (2006):

excommunication
    n 1: the state of being excommunicated [syn: excommunication,
         exclusion, censure]
    2: the act of banishing a member of a church from the communion
       of believers and the privileges of the church; cutting a
       person off from a religious society [syn: excommunication,
       excision]

The Devil's Dictionary (1881-1906):

EXCOMMUNICATION, n.

    This "excommunication" is a word
    In speech ecclesiastical oft heard,
    And means the damning, with bell, book and candle,
    Some sinner whose opinions are a scandal --
    A rite permitting Satan to enslave him
    Forever, and forbidding Christ to save him.
                                                            Gat Huckle