What does "axiom" mean?

Axiom: An established principle in some art or science, which, though not a necessary truth, is universally received; as, the axioms of political economy. An axiom is a self-evident truth which is taken for granted as the basis of reasoning. A maxim is a guiding principle sanctioned by experience, and relating especially to the practical concerns of life. An aphorism is a short sentence pithily expressing some valuable and general truth or sentiment. An adage is a saying of long-established authority and of universal application.