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]

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

EXCOMMUNICATION, eccl. law. An ecclesiastical sentence, pronounced by a
spiritual judge against a Christian man, by which he is excluded from the
body of the church, and disabled to bring any action, or sue any person in
the common law courts. Bac. Ab. h.t.; Co. Litt. 133-4. In early times it
was the most frequent and most severe method of executing ecclesiastical
censure, although proper to be used, said Justinian, (Nov. 123,) only upon
grave occasions. The effect of it was to remove the excommunicated "person
not only from the sacred rites but from the society of men. In a certain
sense it interdicted the use of fire and water, like the punishment spoken
of by Caesar, (lib, 6 de Bell. Gall.). as inflicted by the Druids. Innocent
IV. called it the nerve of ecclesiastical discipline. On repentance, the
excommunicated person was absolved and received again to communion. These
are said to be the powers of binding and loosing the keys of the kingdom of
heaven. This kind of punishment seems to have been adopted from the Roman
usage of interdicting the use of fire and water. Fr. Duaren, De Sacris
Eccles. Ministeriis, lib. 1, cap. 3. See Ridley's View of the Civil. and
Ecclesiastical Law, 245, 246, 249.



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