The Collaborative International Dictionary of English v.0.48:

Betrayal \Be*tray"al\n.
   The act or the result of betraying.
   [1913 Webster]

WordNet (r) 3.0 (2006):

betrayal
    n 1: an act of deliberate betrayal [syn: treachery,
         betrayal, treason, perfidy]
    2: the quality of aiding an enemy

Moby Thesaurus II by Grady Ward, 1.0:

63 Moby Thesaurus words for "betrayal":
   Judas kiss, abuse, apostasy, babbling, backsliding, bad faith,
   blabbering, blabbing, bolt, breach of faith, breakaway,
   communication leak, criminal assault, dead giveaway, debauchment,
   defection, defilement, defloration, deflowering, dereliction,
   deserter, desertion, despoilment, disclosure, disloyalty,
   divulgation, divulgement, divulgence, divulging, double cross,
   evulgation, faithlessness, giveaway, going over, indiscretion,
   leak, letting out, obvious clue, perfidy, priapism, rape, ratting,
   ravage, ravishment, recidivation, recidivism, recreancy,
   revelation, schism, secession, seducement, seduction, sellout,
   sexual assault, telltale, telltale sign, traitorousness, treachery,
   treason, turning traitor, unwitting disclosure, violation,
   walkout