self-defence means fighting off something or another person to protect yourself and maybe others. ways of self-defence include martial arts or using a weapon. sometimes, self-defense can cause serious harm to the other person. in most nations, you cannot be prosecuted for this harm. in the united states, an act of self-defence can only count as self-defence if the victim is in a situation that can cause them serious injury or death. in international law all persons have the right to self-defence.