rule

Etymology 1

From Middle English reule, rewle, rule, borrowed from Old French riule, reule, from Latin regula (“straight stick, bar, ruler, pattern”), from regō (“to keep straight, direct, govern, rule”), ultimately from Proto-Indo-European *h₃réǵeti (“to straighten; right”), from the root *h₃reǵ-; see regent.

noun

1. A regulation, law, guideline.
   All participants must adhere to the rules.
   We profess to have embraced a religion which contains the most exact rules for the government of our lives. a. 1694, John Tillotson, Of The Obligations of Christians to a Holy Life
   The ability to shift profits to low-tax countries by locating intellectual property in them[…]is often assumed to be the preserve of high-tech companies.[…]current tax rules make it easy for all sorts of firms to generate[…]“stateless income”: profit subject to tax in a jurisdiction that is neither the location of the factors of production that generate the income nor where the parent firm is domiciled. 2013-06-22, “T time”, in The Economist, volume 407, number 8841, page 68
2. A regulating principle.
3. The act of ruling; administration of law; government; empire; authority; control.
4. A normal condition or state of affairs.
   My rule is to rise at six o'clock.
   As a rule, our senior editors are serious-minded.
5. (obsolete) Conduct; behaviour.
6. (law) An order regulating the practice of the courts, or an order made between parties to an action or a suit.
7. (mathematics) A determinate method prescribed for performing any operation and producing a certain result.
   a rule for extracting the cube root
8. A ruler; device for measuring, a straightedge, a measure.
   As we may observe in the Works of Art, a Judicious Artist will indeed use his Eye, but he will trust only to his Rule. a. 1716, Robert South, Sermons
9. A straight line (continuous mark, as made by a pen or the like), especially one lying across a paper as a guide for writing.
10. (printing, dated) A thin plate of brass or other metal, of the same height as the type, and used for printing lines, as between columns on the same page, or in tabular work.

Etymology 2

From Middle English reulen, rulen, borrowed from Old French riuler, from Latin regulāre (“to regulate, rule”), from regula (“a rule”); see regular and regulate.

verb

1. (transitive, stative) To regulate, be in charge of, make decisions for, reign over.
2. (slang, intransitive, stative) To excel.
   This game rules!
3. (intransitive) To decide judicially.
   The US supreme court has ruled unanimously that natural human genes cannot be patented, a decision that scientists and civil rights campaigners said removed a major barrier to patient care and medical innovation. 2013-06-21, Karen McVeigh, “US rules human genes can't be patented”, in The Guardian Weekly, volume 189, number 2, page 10
4. (transitive) To establish or settle by, or as by, a rule; to fix by universal or general consent, or by common practice.
5. (transitive) To mark (paper or the like) with rules (lines).

Etymology 3

Related to revel.

noun

1. (obsolete) Revelry.

verb

1. (obsolete, intransitive) To revel.