Source : Webster's Revised Unabridged Dictionary (1913)

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.

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

   2. That which is omitted or is left undone.

Source : WordNet®

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