What legal term is used to describe the defense of using force when one believes it is necessary against unlawful force from another?

The term that describes the defense of using force against unlawful force from another is "self-defense." This legal concept allows an individual to use reasonable force to protect themselves from imminent harm or unlawful aggression. Self-defense is grounded in the belief that it is both necessary and reasonable to respond to threats or attacks in order to safeguard oneself or others from potential injury.

In the context of self-defense, the individual perceives a real or imminent threat and responds in a way that they believe is proportionate to the aggression faced. The law recognizes this instinctual response as a justified reaction, provided that the level of force used is reasonable and necessary given the circumstances.

The other terms listed do not carry the same legal implications as self-defense. Counterforce generally refers to an opposing action but lacks the specific legal framework surrounding necessary force. Retaliation suggests a response to an action that occurred in the past rather than a direct response to an immediate threat. Mitigation is a term more commonly used in legal contexts related to reducing the severity of a situation rather than justifying the use of force. Thus, "self-defense" is the correct term that encapsulates the protective legal doctrine related to using force in response to unlawful aggression.