Omission, n.
Etymology: [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.
Shades of meaning
Fernald has no entry of its own for omission; he lists it under neglect.
He lists it without comment; the entries above explain its neighbors.