The non-aggression principle (NAP), also called the non-aggression axiom, is a concept in which aggression, defined as initiating or threatening any forceful interference with either an individual or their property, is inherently wrong. It is considered by some to be a defining principle of libertarianism in the United States and is also a prominent idea in anarcho-capitalism and minarchism. In contrast to pacifism, the NAP does not forbid forceful defense. There is no single or universal interpretation or definition of the NAP as it faces several definitional issues, including those revolving around intellectual property, force, abortion, and other topics.