Axiom

A self-evident and necessary truth, or a proposition whose truth is so evident as first sight that no reasoning or demonstration can make it plainer; a proposition which it is necessary to take for granted; as, "The whole is greater than a part;" "A thing can not, at the same time, be and not be."

An established principle in some art or science, which, though not a necessary truth, is universally received; as, the axioms of political economy.

(logic) a proposition that is not susceptible of proof or disproof; its truth is assumed to be self-evident