The non-aggression principle (also called the non-aggression axiom, or the anti-coercion or zero aggression principle or non-initiation of force) is 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.
Michael W. Dean's Gun Training with the Non-Aggression Principle, Vol. 1: Basic Handgun and Rifle [DVD] [English]