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