The Collaborative International Dictionary of English v.0.48:

Omission \O*mis"sion\, n. [L. omissio: cf. F. omission. See
   Omit.]
   1. The act of omitting; neglect or failure to do something
      required by propriety or duty.
      [1913 Webster]

            The most natural division of all offenses is into
            those of omission and those of commission.
                                                  --Addison.
      [1913 Webster]

   2. That which is omitted or is left undone.
      [1913 Webster]

Moby Thesaurus II by Grady Ward, 1.0:

203 Moby Thesaurus words for "omission":
   Sisyphean labor, abbreviation, abridgment, absence, arrearage,
   atrocity, bad policy, balk, ban, bar, barring, beggary, bevue,
   blank, blockade, blue-penciling, bowdlerization, boycott, breach,
   break, cancellation, censoring, censorship, chasm, circumscription,
   crime, crime against humanity, culpa, culpable negligence,
   deadly sin, debarment, debarring, defalcation, default, defect,
   defectiveness, deficiency, deficit, deletion, delinquency,
   demarcation, deprivation, dereliction, destitution, disconformity,
   discontinuity, disregard, dropping, drought, editing, elimination,
   embargo, endless task, enormity, erasure, error, evil, exception,
   excision, excluding, exclusion, expurgation, failure, false move,
   false step, famine, fault, felony, gap, genocide, guilty act,
   heavy sin, hiatus, imperfection, impolicy, impoverishment,
   impropriety, inadmissibility, inadvertence, inadvertency,
   inattention, incompleteness, indiscretion, inexecution,
   inexpedience, inexpediency, inexpiable sin, iniquity, injunction,
   injury, injustice, inobservance, interval, laches, lack, lacuna,
   laissez-faire, lapse, lapsus calami, lapsus linguae, laxity,
   laxness, leaving out, lockout, loose ends, loose thread, looseness,
   maladministration, malefaction, malfeasance, malpractice, malum,
   minor wrong, misadministration, misconduct, miscue, misdeed,
   misdemeanor, misdirection, misfeasance, misgovernment, misguidance,
   mishandling, mismanagement, misrule, missing link, misstep,
   mortal sin, narrowing, need, neglect, neglectfulness, negligence,
   nonaccomplishment, nonachievement, nonadherence, nonadmission,
   noncompliance, nonconformance, nonconformity, nonconsummation,
   nondischarging, nonexecution, nonfeasance, nonfulfillment,
   noninterference, nonobservance, nonperformance, nonrestriction,
   offense, omitting, outage, outrage, overlook, overlooking,
   oversight, peccadillo, peccancy, permissiveness, poor stewardship,
   preclusion, preterition, procrastination, prohibition, rejection,
   relegation, remissness, repudiation, restriction, rough edges,
   shortage, shortcoming, shortfall, sin, sin of commission,
   sin of omission, sinful act, skip, skipping, slackness, slight,
   slip, slipup, starvation, striking, stumble, taboo, tort,
   transgression, trespass, trip, ullage, unfulfillment, unobservance,
   unrigorousness, unutterable sin, venial sin, want, wantage,
   work of Penelope, wrong, wrong step, wrongdoing