A statement of what must or must not be. A rule differs from a description in that breaking it has consequence.
Latin regula (a straight edge, a pattern to measure by), from regere (to guide, to keep straight); a rule was first a physical measuring stick before it named a statement whose breaking has consequence.
A statement of what must or must not be, carrying the old measuring-stick's own promise: a rule only holds if breaking it is checked against something straight.