These are some collected attempts at defining “the NAP”.
The initiation of violence against peaceful people is bad.
Pretty Standard?
In simplest terms, the NAP is an ethical stance that 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.
Mises Institute Wiki, ht Godarchy
The non-aggression principle (NAP), also called the non-aggression axiom, the anti-coercion, zero aggression principle, or non-initiation of force, is an ethical stance asserting that aggression is inherently wrong. In this context, aggression is defined as initiating or threatening any forceful interference with an individual or their property.[1] In contrast to pacifism, it does not forbid forceful defense.
Wikipedia
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.
Learn Liberty
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.