The relationship between an act and the consequences it produces. It is one of the elements that must be proved before an accused can be convicted of a crime in which the effect of the act is part of the definition of the crime (e.g. murder). Whatever other causes may have effected the bringing about of the actus reus, it needs to be shown that the defendant's behaviour did, in a significant way, contribute to the actus reus. In R v White  2 KB 124, White gave poison to his mother who died. However, medical evidence proved that the mother had died from a heart attack and that the poison was in no way connected to the death. Therefore the defendant's behaviour did not contribute in any way to the resulting death.
Causation is a question of both 1) fact and 2) law and in both cases this is a question for the jury to decide:
In tort it must be established that the defendant's tortious conduct caused or materially contributed to the damage to the claimant before the defendant can be found liable for that damage. In order to determine factual causation, courts adopt the same “but for” test used in criminal cases: “but for” the defendant's tortious conduct, would the claimant's loss have occurred (Barnett v Chelsea and Kensington Hospital Management Committee  1 QB 428)? However, this test is inadequate for cases of concurrent or cumulative causes, where the actions of two or more tortfeasors are each sufficient to produce the damage. Where there is more than one possible cause there are several tests that may be applied. For example, did the defendant's negligence materially increase the risk of injury (Mc Ghee v National Coal Board  1 WLR 1)? Similarly, was the defendant's breach of duty a material cause of the injury? One of five causes will not suffice to establish liability (Wilsher v Essex Area Health Authority  AC 1074). In order for causation to be established on the balance of probabilities, it requires at least a 51% chance that the defendant's actions caused the damage (Hotson v East Berkshire Health Authority  AC 750).