The Collaborative International Dictionary of English v.0.48:

Non-feasance \Non-fea"sance\, n. [Pref. non- + OF. faisance a
   doing, fr. faire to do.] (Law)
   An omission or neglect to do something, esp. that which ought
   to have been done. Cf. Malfeasance.
   [1913 Webster]

WordNet (r) 3.0 (2006):

nonfeasance
    n 1: a failure to act when under an obligation to do so; a
         refusal (without sufficient excuse) to do that which it is
         your legal duty to do

Moby Thesaurus II by Grady Ward, 1.0:

111 Moby Thesaurus words for "nonfeasance":
   Sisyphean labor, atrocity, bad policy, breach, crime,
   crime against humanity, culpa, culpable negligence, deadly sin,
   default, delinquency, dereliction, disconformity, disregard,
   endless task, enormity, error, evil, failure, fault, felony,
   genocide, guilty act, heavy sin, impolicy, impropriety,
   inadvertence, inadvertency, inattention, indiscretion, inexecution,
   inexpedience, inexpediency, inexpiable sin, iniquity, injury,
   injustice, inobservance, laches, laissez-faire, lapse, laxity,
   laxness, loose ends, looseness, maladministration, malefaction,
   malfeasance, malpractice, malum, minor wrong, misadministration,
   misconduct, misdeed, misdemeanor, misdirection, misfeasance,
   misgovernment, misguidance, mishandling, mismanagement, misrule,
   mortal sin, neglect, neglectfulness, negligence, nonaccomplishment,
   nonachievement, nonadherence, noncompliance, nonconformance,
   nonconformity, nonconsummation, nondischarging, nonexecution,
   nonfulfillment, noninterference, nonobservance, nonperformance,
   nonrestriction, offense, omission, outrage, overlooking, oversight,
   peccadillo, peccancy, permissiveness, poor stewardship,
   procrastination, remissness, rough edges, sin, sin of commission,
   sin of omission, sinful act, slackness, slight, slip, tort,
   transgression, trespass, trip, unfulfillment, unobservance,
   unrigorousness, unutterable sin, venial sin, work of Penelope,
   wrong, wrongdoing