ruler

Etymology

From Middle English reuler; equivalent to rule + -er.

noun

1. A (usually rigid), flat, rectangular measuring or drawing device with graduations in units of measurement; a straightedge with markings.
2. A person who rules or governs; someone or something that exercises dominion or controlling power over others.
   The carcharhinids are unquestionably the rulers of the seas, because of their abundance, their strength and speed, and their unchallenged position as the number one predators in the marine ecosystem. 1983, Richard Ellis, The Book of Sharks, Knopf, page 122
   LIKE THE MANDARINS of old, the rulers of China live behind high walls. When they emerge, which they rarely do, they travel in cars with rear windows curtained like sedan chairs. They live in the Chung Nan Hai, a walled park adjacent to the Forbidden City from where ancient dynasties ruled the Celestial Empire. 1991, Chris Mullin, The Year of the Fire Monkey (Fiction), London: Chatto & Windus, →OCLC, page 252

verb

1. (transitive) To beat with a ruler (as a traditional school punishment).