The Collaborative International Dictionary of English v.0.48:

Defamation \Def`a*ma"tion\, n. [OE. diffamacioun, F.
   diffamation. See Defame.]
   Act of injuring another's reputation by any slanderous
   communication, written or oral; the wrong of maliciously
   injuring the good name of another; slander; detraction;
   calumny; aspersion.
   [1913 Webster]

   Note: In modern usage, written defamation bears the title of
         libel, and oral defamation that of slander. --Burrill.
         [1913 Webster]

WordNet (r) 3.0 (2006):

defamation
    n 1: a false accusation of an offense or a malicious
         misrepresentation of someone's words or actions [syn:
         defamation, calumny, calumniation, obloquy,
         traducement, hatchet job]
    2: an abusive attack on a person's character or good name [syn:
       aspersion, calumny, slander, defamation,
       denigration]

Moby Thesaurus II by Grady Ward, 1.0:

24 Moby Thesaurus words for "defamation":
   attack, backbiting, backstabbing, belittlement, blackening,
   calumny, character assassination, defamation of character,
   defilement, denigration, depreciation, disparagement,
   malicious defamation, muckraking, mudslinging, name-calling,
   revilement, scandal, slander, smear, smear campaign, smear word,
   tale, vilification



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

DEFAMATION, tort. The speaking slanderous words of a person so as, de bona
fama aliquid detrahere, to hurt his good fame. Vide Slander.
     2. In the United States, the remedy for defamation is by an action on
the case, where the words are slanderous.
     3. In England, besides the remedy by action, proceedings may be
instituted in the ecclesiastical court for redress of the injury. The
punishment for defamation, in this court, is payment of costs and penance
enjoined at the discretion of the judge. When the slander has been privately
uttered, the penance may be ordered to be performed in a private place; when
publicly uttered, the sentence must be public, as in the church of the
parish of the defamed party, in time of divine service,, and the defamer may
be required publicly to pronounce that by such words, naming them, as set
forth in the sentence, he had defamed the plaintiff, and, therefore, that he
begs pardon, first, of God, and then of the party defamed, for uttering such
words. Clerk's Assist. 225; 3 Burn's Eccl. Law, Defamation, pl. 14; 2 Chit.
Pr. 471 Cooke on Def.