The Collaborative International Dictionary of English v.0.48:

Nonobservance \Non`ob*serv"ance\ (n[o^]n`[o^]b*z[~e]rv"ans), n.
   Neglect or failure to observe or fulfill.
   [1913 Webster]

WordNet (r) 3.0 (2006):

nonobservance
    n 1: a lack of conformity with law or custom or practice etc.
         [ant: honoring, observance]

Moby Thesaurus II by Grady Ward, 1.0:

96 Moby Thesaurus words for "nonobservance":
   Laodiceanism, abnegation, absentmindedness, carelessness,
   contradiction, contrariety, declension, declination, declinature,
   declining, denial, deprivation, deviation, disaccord,
   disaccordance, disagreement, disallowance, disclaimer,
   disclamation, disconformity, disobedience, disregard,
   disregardfulness, dissent, distraction, flightiness, flippancy,
   frivolousness, giddiness, heedlessness, holding back, inaccordance,
   inadvertence, inadvertency, inattention, inattentiveness,
   incongruity, inconsideration, inconsistency, incuriosity,
   indevotion, indevoutness, indifference, indifferentism,
   inobservance, levity, lightmindedness, lukewarm piety, nay,
   negation, negative, negative answer, negligence, nix, no,
   nonacceptance, noncompliance, nonconcurrence, nonconformance,
   nonconformism, nonconformity, nonconsent, nonreligiousness,
   obliviousness, originality, protest, recalcitrance, recantation,
   recusance, recusancy, refractoriness, refusal, regardlessness,
   rejection, repudiation, retention, shallowness, superficiality,
   thoughtlessness, thumbs-down, turndown, unalertness, unawareness,
   unconformity, unconsciousness, undevoutness, undutifulness,
   unheedfulness, unintentiveness, unmindfulness, unobservance,
   unreligiousness, unwariness, unwatchfulness, unwillingness,
   withholding