Walter Block writes:
The non-aggression [principle] ...is the lynchpin of the philosophy of libertarianism. It states, simply, that it shall be legal for anyone to do anything he wants, provided only that he not initiate (or threaten) violence against the person or legitimately owned property of another. That is, in the free society, one has the right to manufacture, buy or sell any good or service at any mutually agreeable terms. Thus, there would be no victimless crime prohibitions, price controls, government regulation of the economy, etc.
The above originally appeared at LewRockwell.com.
Note: Dr. Block identifies NAP as the non-aggression axiom. I have altered the definition to align with my view that it is a (man-made) principle rather than an axiom.