There are many historical antecedents to the NAP, but libertarians usually trace its current formulation to Murray Rothbard, who put it as follows:

“The fundamental axiom of libertarian theory is that no one may threaten or commit violence (“aggress”) against another man’s person or property. Violence may be employed only against the man who commits such violence; that is, only defensively against the aggressive violence of another. In short, no violence may be employed against a nonaggressor. Here is the fundamental rule from which can be deduced the entire corpus of libertarian theory.“

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. The principle is a deontological (or rule-based) ethical stance.

The non-aggression principle (NAP), also called the non-aggression axiom, is the fundamental principle of morality that states that any person is permitted do everything with his property except aggression, defined as the initiation of forceful action, which is in turn defined as 'the application or threat of' 'physical interference (property breach) or fraud (contract breach)', any of which without consent. The principle is also called the non-initiation of force. The principle incorporates universal enforceability.