The Collaborative International Dictionary of English v.0.48:

Imprecation \Im`pre*ca"tion\, n. [L. imprecatio: cf. F.
   impr['e]cation.]
   The act of imprecating, or invoking evil upon any one; a
   prayer that a curse or calamity may fall on any one; a curse.
   [1913 Webster]

         Men cowered like slaves before such horrid
         imprecations.                            --Motley.

   Syn: Malediction; curse; execration; anathema. See
        Malediction.
        [1913 Webster]

WordNet (r) 3.0 (2006):

imprecation
    n 1: the act of calling down a curse that invokes evil (and
         usually serves as an insult); "he suffered the imprecations
         of the mob" [syn: imprecation, malediction]
    2: a slanderous accusation

Moby Thesaurus II by Grady Ward, 1.0:

44 Moby Thesaurus words for "imprecation":
   adjuration, anathema, appeal, application, ban, beseechment, bid,
   blasphemy, call, clamor, commination, cry, curse, cursing, cussing,
   damnation, denunciation, entreaty, evil eye, excommunication,
   execration, fulmination, hex, imploration, imploring, invocation,
   invocatory plea, malediction, malison, malocchio, obsecration,
   obtestation, orison, petition, plea, prayer, profanity,
   proscription, rogation, suit, supplication, swearing, thundering,
   whammy