The Collaborative International Dictionary of English v.0.48:

Disloyalty \Dis*loy"al*ty\, n. [Pref. dis- + loyalty: cf. OF.
   desloiaut['e], deslealt['e], F. d['e]loyaut['e].]
   Want of loyalty; lack of fidelity; violation of allegiance.
   [1913 Webster]

WordNet (r) 3.0 (2006):

disloyalty
    n 1: the quality of being disloyal [ant: loyalty, trueness]

Moby Thesaurus II by Grady Ward, 1.0:

39 Moby Thesaurus words for "disloyalty":
   Punic faith, apostasy, backsliding, bad faith, barratry, betrayal,
   bolt, breach of faith, breach of promise, breach of trust,
   breakaway, defection, dereliction, desertion, disaffection,
   faithlessness, falseness, falsity, fickleness, going over,
   inconstancy, infidelity, mala fides, perfidiousness, perfidy,
   ratting, recidivation, recidivism, recreancy, secession,
   treacherousness, treason, trothlessness, turning traitor, unfaith,
   unfaithfulness, unloyalty, unsteadfastness, untrueness