A libertarian is a person who believes that no one has the right, under any circumstances, to initiate force against another human being, or to advocate or delegate its initiation.
Those who act consistently with this principle are libertarians, whether they realize it or not. Those who fail to act consistently with it are not libertarians, regardless of what they may claim.

(with Rylla Cathryn Smith) What Libertarians Believe, "Introduction: The Zero Aggression Principle"