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]

WordNet (r) 3.0 (2006):

omission
    n 1: a mistake resulting from neglect [syn: omission, skip]
    2: something that has been omitted; "she searched the table for
       omissions"
    3: any process whereby sounds or words are left out of spoken
       words or phrases [syn: omission, deletion]
    4: neglecting to do something; leaving out or passing over
       something

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



Bouvier's Law Dictionary, Revised 6th Ed (1856):

OMISSION. An omission is the neglect to perform what the law requires.
     2. When a public law enjoins on certain officers duties to be performed
by them for the public, and they omit to perform them, they may be indicted:
for example, supervisors of the highways are required to repair the public
roads; the neglect to do so will render them liable to be indicted.
     3. When a nuisance arises in consequence of an omission, it cannot be
abated if it be a private nuisance without giving notice, when such notice
can be given. Vide Branches; Commission; Nuisance; Trees.