Non-aggression Principle – an ethical stance which asserts that aggression is inherently illegitimate. “Aggression” is defined as the “initiation” of physical force against persons or property, the threat of such, or fraud upon persons or their property. In contrast to pacifism, the non-aggression principle does not preclude violent self-defense.

Source:  http://wiki.mises.org/wiki/Principle_of_non-aggression