Actus reus, means the wrongful deed that comprises the physical components of a crime and that generally must be coupled with mens rea to establish criminal liability; a forbidden act i.e. the actus reus for theft is the taking of or unlawful control over property without the owner's consent. Also termed deed of crime; overt act, Black Law Dictionary, 7th Edn., p. 37.
The phrase 'deed of crime' i.e. actus reus as so used does not indicate the crime itself but merely one of the ingredients of crime; and this ingredient may be present without any crime at all, just as hydrogen is one of the ingredients of water but may be present without water. The words 'deed of crime' are so suggesting of the crime itself, however, that perhaps the Latin phrase 'actus reus' is less likely to cause confusion. The actus reus is essential to crime but is not sufficient for this purpose without the necessary mens rea, just as mens rea is essential to crime but is sufficient without the necessary 'actus reus'. Rollin M. Perkins & Ronald N. Boyce, Criminal Law 831, 3rd Edn., 1982.
The word actus connotes a 'deed', a physical result of human conduct. When criminal policy regards such a deed as sufficiently harmful it prohibits it and seeks to prevent its occurrence by imposing a penalty for its commission. It has long been the custom of lawyers to describe a deed so prohibited by law in the words actus reus. Thus actus reus may be defined as 'such result of human conduct as the law seeks to prevent'. It is important to note that the actus reus, which is the result of conduct, and therefore an event, must be distinguished from the conduct which produced the result. For example, in a simple case of murder it is the victim's death (brought about by the conduct of the murderer) which is the actus reus in the mens rea is the murderer's intention to cause that death. In other words, the crime is constituted by the event, and not by the activity (or in certain cases as we shall see, by the omission to act) which caused the event. J.W. Cecil Turner, Kenny's Outlines of Criminal Law, 13 (16th Edn. 1952).