The non-aggression principle (also called the non-aggression axiom, the anti-coercion principle, the zero aggression principle, the non-initiation of force), or NAP for short, is a moral stance which asserts that aggression is inherently illegitimate. Aggression, for the purposes of the NAP, is defined as the initiation or threatening of violence against a person or legitimately owned property of another. Specifically, any unsolicited actions of others that physically affect an individual’s property, including that person’s body, no matter if the result of those actions is damaging, beneficiary or neutral to the owner, are considered violent when they are against the owner’s free will and interfere with his right to self-determination, as based on the libertarian principle of self-ownership. Supporters of NAP use it to demonstrate the immorality of theft, vandalism, assault, and fraud. In contrast to pacifism, the non-aggression principle does not preclude violence used in self-defense or defense of others.
http://en.wikipedia.org/wiki/Non-aggression_principle