Axiom

An axiom is any starting assumption from which other statements are logically derived. It can be a sentence, a proposition, a statement or a rule that forms the basis of a formal system. Unlike theorems, axioms cannot be derived by principles of deduction nor demonstrable by formal proofs—simply because they are starting assumptions—there is nothing else they logically follow from (otherwise they would be called theorems). In many contexts, "axiom," "postulate," and "assumption" are used interchangeably.