Axiom
Etymology
From Greek "axiōma," "that which is thought worthy or fit," and hence "a self-evident principle," from "axios," "worthy." In Euclid’s geometry the axioms were the unproven bedrock from which every theorem was derived. The word still names the assumptions a system takes as given — and reminds you that the deepest disagreements often live not in the arguments but in the axioms beneath them.
noun · /AK-see-um/
A statement accepted as self-evidently true and used as a starting point for further reasoning.
What it names
The foundation you build on without proving — worth surfacing because whole systems of belief rest on axioms nobody in the room has actually examined.
Example
The whole strategy rested on the unstated axiom that customers would always prefer the cheaper option.
Related idea
An axiom is assumed without proof at the base of a system; a corollary sits at the other end, claiming nothing on its own but following automatically once the work is done.