Source : Webster's Revised Unabridged Dictionary (1913)

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.

Source : WordNet®

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