An omission is a failure to act or disclose something that should have been done or said. In law, an omission is generally treated differently from positive conduct and can attract different legal consequences). In criminal law, an omission can constitute an actus reus and give rise to liability only when the law imposes a duty to act). For example, if a person fails to take measures that lie within their power to counteract a danger that they have created, they may be held criminally liable).

In legal disputes, omissions can become a tricky business, as they can endanger oneself and/or others. For example, failing to report a car incident can be a criminal charge. There are some omissions that are chargeable, and some that are not. In serious cases, punishments can even result in prison sentences, such as manslaughter due to an omission.

Examples of omissions include neglecting to do something that should have been done, such as failing to help someone in need or failing to disclose important information. Omissions can also refer to something that has been left out or neglected, such as a missing piece of information in a report.