"The libertarian creed rests upon one central axiom: that no man or group of men may aggress against the person or property of anyone else. This may be called the 'nonaggression axiom.' 'Aggression' is defined as the initiation of the use or threat of physical violence against the person or property of anyone else. Aggression is therefore synonymous with invasion."
Murray N. Rothbard, For a New Liberty. The Libertarian Manifesto
The quote above is from chapter 2: "Property and Exchange".
The whole book can be read at:
http://www.mises.org/rothbard/foranewlb.pdf (pdf version)
Polski przekład drugiego rozdziału "Manifestu libertariańskiego":