Intent is a concept at common law. In criminal law, it establishes mens rea and, more importantly, is used to determine whether mens rea actually exists, even when the results of a crime are not what the defendant actually meant to happen. In tort law, it is an essential element in establishing most torts (the primary exception being negligence).
The concept of intent poses difficulty for most laypersons, and can even give judges pause. It is important to remember that, when used in a legal context, it cannot be given it's ordinary English meaning. It should not be confused with motive even though in normal English usage these two are close synonyms. The maxim that best describes intent in a legal sense is "Intent follows the bullet, not the aim".
In other words, in both establishing a crime or a tort, the defendant is held to have intent whenever something happens as a result of the defendant's deliberate actions as long as that result was reasonably foreseeable. A pair of examples can make this clear.
- Bill wants to kill Harry because Harry has been sleeping with his girlfriend. He fires a gun at Harry but misses and hits his best friend George. Bill is still guilty of murder even though he did not want to kill George. Bill's intent to kill Harry forms the necessary mens rea.
- After Bill is released, he still wants to punish Harry, but decides this time only to wound him. He shoots Harry in the leg, but hits the femoral artery and Harry soon bleeds to death. Bill is still guilty of murder and not the offence he wanted to commit, assault with a weapon, because intent attaches to the act of shooting, not Bill's wish to merely wound Harry. Since bleeding to death from a leg wound is a foreseeable consequence of being shot, that forms the necessary mens rea for the more serious offence.
Things get more complicated when considering whether a person is guilty of attempting a crime. Let's take another example - tired of prison, Bill decides to escape. He is confronted by a guard on a high wall. Although he didn't want to hurt the guard, he has to push him off the wall to make his escape. Remarkably, the guard survives the fall. Had the guard died, we clearly have murder again, but with the guard surviving, it is not as clear whether Bill had the required intent for a charge of attempted murder.