Actus reus refers to the physical act or conduct that constitutes a criminal offense. It is one of the essential elements of a crime, alongside mens rea, which is the mental state or intent behind the act. The actus reus can include actions, omissions, or a state of being that leads to a crime. To establish criminal liability, it must be proven that the actus reus occurred. This means showing that the defendant engaged in a voluntary action or failed to act when there was a legal duty to do so. Understanding actus reus is crucial in the field of criminal law to determine accountability for criminal behavior.