Self-defense is a possible justification for what would otherwise be breaking the law. Defendants who use this defense argue that they should not be held liable for what is normally a crime, since the actionss taken were intended to protect the defendant or others from danger.
Self-defense usually refers to the use of violence to protect oneself; this forms the basis for many of the martial arts. Self-defense is generally understood to have two degrees: reasonable force (or non-lethal force) and deadly force. Courts generally find that the violence a defendant uses, in self-defense, must in some fashion be comparable to the threat faced, so that deadly force should only be used in situations of "extreme" danger. Many courts have ruled that a "path of retreat" exists, and that self-defense is only acceptable, as a legal defense, when there was insufficient opportunity for the defendant to flee. However, in defense of property, the "castle exception" (see: Edward Coke) argues that one cannot be expected to retreat from one's own home.
Courts tend to rule against the right of an individual to "preemptively" engage in self-defense. For example, victims of domestic violence are generally not considered justified in murdering an individual whom they believe will someday attack with lethal force.