What is Omission?

Legal Definition
An omission is a failure to act, which generally attracts different legal consequences from positive conduct. In the criminal law, an omission will constitute an actus reus and give rise to liability only when the law imposes a duty to act and the defendant is in breach of that duty. In tort law, similarly, liability will be imposed for an omission only exceptionally, when it can be established that the defendant was under a duty to act.
-- Wikipedia
Legal Definition
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.
-- Bouviers Law Dictionary
Legal Definition
Failure to complete a duty or task, usually as a result of apathy, complacency or neglect.
Legal Definition
The failure to perform some act or duty.
-- Ballentine's Law Dictionary