An axiom is a self-evident truth. The authors of the Declaration of Independence could have written, “We hold these truths to be axiomatic,” but it wouldn't have the same ring.
The root word of axiomatic, axiom, derives from the Greek axioma, meaning "authority," or "that which is thought worthy or fit." We use it to describe statements that have the authority of truth about them, or that seem worthy of the truth, or fit to be described as such. That is, an axiom is a proposition that we don’t generally question because it seems plain enough that it’s true. And axiomatic means evident without proof or argument.