Postulate
A statement, also known as an axiom, which is taken to be true without proof. Postulates are the basic structure from which lemmas and theorems are derived. The whole of Euclidean geometry, for example, is based on five postulates known as Euclid's postulates.
See also
Archimedes' Axiom, Axiom, Bertrand's Postulate, Conjecture, Equidistance Postulate, Euclid's Postulates, Lemma, Parallel Postulate, Porism, Proof, Theorem, Triangle PostulateExplore with Wolfram|Alpha
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Weisstein, Eric W. "Postulate." From MathWorld--A Wolfram Resource. https://mathworld.wolfram.com/Postulate.html