Definition - What does Self-Defense mean?
Self-defense is the act of protecting oneself or the life of another from someone who threatens the safety of the vulnerable person(s).
In the United States, the right to self-defense is protected in the Constitution and includes, but is not limited to, the right to shoot a person who is trespassing or otherwise legally threatening the safety of another. It is a commonly used defense.
Martial arts training is a common part of self-defense, as is training in the safe use of firearms. There is no set definition as to what weapon can be used for self-defense; the important thing is that the action could later be deemed to be necessary at the time.
Justipedia explains Self-Defense
In the U.S., it is legal for a person to use deadly force for the purpose of self-defense. A person who uses self-defense is responsible for only using the amount of force (reasonable force) that would be needed to allow that person and/or any other vulnerable person to reach safety. This means that a person should only use deadly force for self-defense as a last resort. In many other countries in the world, the ability to use self-defense to the point of deadly force is not allowed.