Postulate

The term postulate, or axiom, indicates a statement or assumption that is agreed by everyone to be so obvious or self-evident that no proof is necessary, and which can be used to prove other statements or theorems. Neither axioms nor postulates can be proven (within a system) using more basic statements. However, in many elementary textbooks, where the student does not have the training to understand a more rigorous approach, many otherwise-provable statements are accepted as postulates to allow further development of the subject.