Actus reus: the prohibited act.
For example, in Canada's Criminal Code, at s. 322, the actus reus of theft is defined as follows:
"Every one commits theft who fraudulently and without colour of right takes, or fraudulently and without colour of right converts to his use or to the use of another person, anything, whether animate or inanimate, with intent
(a) to deprive, temporarily or absolutely, the owner of it, or a person who has a special property or interest in it, of the thing or of his property or interest in it;
(b) to pledge it or deposit it as security;
(c) to part with it under a condition with respect to its return that the person who parts with it may be unable to perform; or
(d) to deal with it in such a manner that it cannot be restored in the condition in which it was at the time it was taken or converted."
Usually, but not always, the actus reus is defined as an act but an omission can constitute one as well, such as at s. 215, failing "to provide necessaries of life for a child under the age of sixteen years".
During the normal course of a criminal trial, a prosecutor proves actus reus and mens rea beyond a reasonable doubt and barring a successful defence, conviction follows.
The term was judicially considered in R v Théroux 1993 2 SCR 5 where Justice McLachlin, in reference to the actus reus of fraud:
"The offence has two elements: dishonest act and deprivation. The dishonest act is established by proof of deceit, falsehood or 'other fraudulent means'. The the element of deprivation is established by proof of detriment, prejudice, or risk of prejudice to the economic interests of the victim, caused by the dishonest act."