The right of self-defense (also called, when it applies to the defense of another, alter ego defense, defense of others, defense of a third person) is the right for people to use reasonable force or defensive force, for the purpose of defending one's own life (self-defense) or the lives of others, including –in certain circumstances– the use of deadly force.

If a defendant uses defensive force because of a threat of deadly or grievous harm by the other person, or a reasonable perception of such harm, the defendant is said to have a "perfect self-defense" justification. If defendant uses defensive force because of such a perception, and the perception is not reasonable, the defendant may have an "imperfect self-defense" as an excuse.