Assume something or present it as a fact and you postulate it. Physicists postulate the existence of parallel universes, which is a little mind-blowing.
Anyone who has suffered through geometry class is familiar with some of the greatest hits, like Euclid's postulate and the point-line-plane postulate. Those are propositions that have to be assumed for other mathematical statements to follow logically. As a verb (pronounced "POST-you-late") it describes the act of presenting an idea, theory, belief, or concept.