the act of defending one's person when physically attacked, as by countering blows or overcoming an assailant

the art of self-defense.

a claim or plea that the use of force or injuring or killing another was necessary in defending one's own person from physical attack

He shot the man who was trying to stab him and pleaded self-defense at the murder trial.

an act or instance of defending or protecting one's own interests, property, ideas, etc., as by argument or strategy.