Actus reus is the Latin for ‘guilty act’, and is the basis of criminal law in most jurisdictions. Before a court can punish a guilty act, evidence and irrefutable proof have to be presented.

Usually, an act under actus reus means some form of physical crime was committed, as in most societies you can’t be put on trial for thinking criminal thoughts. However, actus reus can also apply if inaction took place or the threat of action was made. Murder is clearly a guilty act, but so is the neglect of a child by its parents, or the threat of violence in domestic disturbances.

It should be noted that even though criminal thoughts cannot be punished in a court of law, words could, as they could be defined as a criminal act, and would, therefore, be applicable. One good example here is perjury, in which the words of the perjurer constitute a criminal act. Another example is solicitation where a criminal act may follow.

Common law will only allow criminal liability if both actus reus and mens rea (the intent to commit the crime) can be proved. The terms originate from Edward Coke, a prominent English jurist in the seventeenth century who held the posts of Attorney General under Queen Elizabeth I and Lord Chief Justice under James I, who stated: "an act does not make a person guilty unless (their) mind is also guilty". This ensures that the criminal intentions actually resulted in the criminal act that the court has been asked to rule on.

In law this means that the person accused of the crime intended to commit the crime. This is different to motive under common law jurisdictions. It is important to separate the two, as criminal intent can often have unintended consequences that may appear to be the motivation for the crime in question when in fact they are not.

Actus reus is a central component of the English criminal legal system. As most crimes include an element of physicality and intention, this principle - together with mens rea - forms the basis on which the case is built against the defendant. In some cases the intention to commit the crime may not be conspicuous, but a link between the defendant’s guilty mind and their subsequent actions must be proven beyond reasonable doubt for a conviction to take place.