Right of self-defense is the right for civilians acting on their own behalf to engage in a level of violence, called reasonable force or defensive force, for the sake of defending one's own life or the lives of others, including, in certain circumstances, the use of deadly force.
Two kinds :
The Real SD is the one you repel an aggression that is already started or when the offending agent has started to attack.
Putative LD, as the name itself tells, you defend something that has not happened yet. The attacker does not initiate any aggressive move, but it is assumed he will.