Actus reus is a criminal act. Most crimes have two components: mens rea (or intent) and actus reus (action). Both intent and action are normally required to find a person guilty of a crime under US Criminal law doctrines.
This term is Latin, and means "the guilty act." Actus reus, by definition, means that there must be an action or physical movement. In other words, it is the overt physical action that makes a person liable for a crime. This is always required in the United States for the commission of a crime, since a person cannot be found guilty of thinking criminal thoughts or wishing that a crime would take place.
Common law defines an act as a voluntary or involuntary bodily movement. Nevertheless, under the Model Penal Code, which has been adopted in whole or in part into the criminal law of many US states, the act must be voluntary. It excludes from punishment acts that are caused by a reflex or convulsion, acts that occurred while unconscious or asleep, acts that occurred under hypnosis, and other bodily movements that are not intended by the actor. These exclusions protect a person from being found guilty of a crime based on action that was not under his or her control.
The concept of actus reus is derived from English common law. Under these principals, an act alone did not make a person guilty of committing a crime — both the illegal action and the illegal intent were required. These principals are important, since a person must both intend to commit a crime and actually take steps toward committing the crime in order to be guilty in the eyes of the law.
An act doesn't necessarily have to be a physical action, but can also be an omission or the act of possessing something. For example, if a person is required to act under the law and fails to do so, the failure can be what is required for a crime. Voluntarily being in possession of an illegal substance, such as a drug, can also be considered an "action."
Under certain circumstances, an act can be made criminal because of the intent behind it. For example, mailing a letter is normally not a criminal action, but if the letter is mailed as part of conspiracy to commit fraud, it can become an illegal action, and constitute the actus reus for the crime.