Definition of "ruler"

ruler

/ˈɹuːlə(ɹ)/

noun
plural rulers
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.
  Quotations
  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, page 252
verb
third-person singular simple present rulers, present participle rulering, simple past and past participle rulered
1. (transitive) To beat with a ruler (as a traditional school punishment).