The Collaborative International Dictionary of English v.0.48:

Treachery \Treach"er*y\, n. [OE. trecher["i]e, trichere, OF.
   trecherie, tricherie, F. tricherie trickery, from tricher to
   cheat, to trick, OF. trichier, trechier; probably of Teutonic
   origin. See Trickery, Trick.]
   Violation of allegiance or of faith and confidence;
   treasonable or perfidious conduct; perfidy; treason.
   [1913 Webster]
   [1913 Webster]

         Be ware, ye lords, of their treachery.   --Chaucer.
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         In the council chamber at Edinburgh, he had contracted
         a deep taint of treachery and corruption. --Macaulay.
   [1913 Webster] Treachetour

WordNet (r) 3.0 (2006):

treachery
    n 1: betrayal of a trust [syn: perfidy, perfidiousness,
         treachery]
    2: an act of deliberate betrayal [syn: treachery, betrayal,
       treason, perfidy]