The definition of "a person justified in using force against another when he reasonably believes the force is necessary to protect himself against unlawful force" is known as_________.

The definition describes a situation in which an individual has the right to use force to defend themselves from unlawful aggression. This principle is known as self-defense. It encapsulates the idea that a person must have a reasonable belief that their actions are necessary to prevent harm to themselves from an aggressor.

Self-defense laws vary by jurisdiction, but they generally require that the force used must be proportional and reasonable compared to the threat faced. This definition emphasizes the necessity of the individual's belief in the requirement of force for protection, which is a key component of self-defense justifications.

The other terms, while related, do not capture the specific legal framework outlined. "Justifiable force" is broader and could encompass various forms of permissible force, not solely limited to the context of self-defense. "Defense of necessity" typically refers to actions taken to prevent a greater harm, which isn't explicitly focused on personal protection from unlawful force. Lastly, "protection" is a vague term that does not convey the same legal implications or conditions necessary for self-defense. Thus, the best fit for the definition provided is self-defense.