An axiom is a statement that everyone believes is true, such as "supply equals demand" or "the only constant is change." Mathematicians use axiom to refer to established proofs.
Many axioms are so widely used they become clichés—here's one to help you remember what the word means. Axioms are so widely accepted they're seen almost as facts. A police officer interrogating a witness might just as easily say, "Stick to the axioms," as "Stick to the facts, ma'am."