The leader of a country is a ruler. If you're a queen, a sultan, or a czar, you're a ruler.

Besides "person who rules or governs," you can define ruler as a measuring device that's marked with inches or centimeters. You can measure the diameter of a circle with a ruler, and you can also draw a perfectly straight line using its edge. Both meanings stem from the verb rule, "to exercise power" or "to control," which came to also mean "mark with lines" in the 1590s.

Definitions of ruler
  1. noun
    a person who rules or commands
    synonyms: swayer
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    Mongolian ruler of Samarkand who led his nomadic hordes to conquer an area from Turkey to Mongolia (1336-1405)
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    a ruler of the Incas (or a member of his family)
    a ruler of the eastern Roman Empire
    a member of the European royal family that ruled France
    calif, caliph, kalif, kaliph, khalif, khalifah
    the civil and religious leader of a Muslim state considered to be a representative of Allah on earth
    dictator, potentate
    a ruler who is unconstrained by law
    ameer, amir, emeer, emir
    an independent ruler or chieftain (especially in Africa or Arabia)
    the ruler of a province (as in the Roman Empire and Byzantine Empire) or certain religious rulers with secular authority
    a Muslim ruler or governor or judge
    any of the British rulers who were members of the House of Hanover
    a title given to rulers or other important people in Asian countries
    Moghul, Mogul
    a member of the Muslim dynasty that ruled India until 1857
    one of the rulers in an oligarchy
    lord, master, overlord
    a person who has general authority over others
    Pharaoh, Pharaoh of Egypt, pharaoh
    the title of the ancient Egyptian kings
    puppet leader, puppet ruler
    a leader or ruler who is chosen by a despot to head a government
    someone who rules during the absence or incapacity or minority of the country's monarch
    a hereditary ruler
    Arab chief, sheik, sheikh, tribal sheik, tribal sheikh
    the leader of an Arab village or family
    crowned head, monarch, sovereign
    a nation's ruler or head of state usually by hereditary right
    a member of the royal family that ruled Scotland and England
    grand Turk, sultan
    the ruler of a Muslim country (especially of the former Ottoman Empire)
    a member of the dynasty that ruled England
    in ancient Greece, a ruler who had seized power without legal right to it
    a member of the Capetian dynasty
    Carlovingian, Carolingian
    a member of the Carolingian dynasty
    Cheops, Khufu
    Egyptian Pharaoh of the 27th century BC who commissioned the Great Pyramid at Giza
    czar, tsar, tzar
    a male monarch or emperor (especially of Russia prior to 1917)
    the male ruler of an empire
    feudal lord, seigneur, seignior
    a man of rank in the ancient regime
    Huayna Capac
    the Incan ruler under whom the Incan empire reached its widest extent (died in 1525)
    Rex, king, male monarch
    a male sovereign; ruler of a kingdom
    a member of the Merovingian dynasty
    Shah, Shah of Iran, shah
    title for the former hereditary monarch of Iran
    a hereditary military dictator of Japan; the shoguns ruled Japan until the revolution of 1867-68
    a powerful political figure who rules by the exercise of force or violence
    autocrat, despot, tyrant
    a cruel and oppressive dictator
    a military dictator in a Spanish-speaking country
    type of:
    individual, mortal, person, somebody, someone, soul
    a human being
  2. noun
    measuring stick consisting of a strip of wood or metal or plastic with a straight edge that is used for drawing straight lines and measuring lengths
    synonyms: rule
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    carpenter's rule
    a rule used by a carpenter
    foot rule
    a ruler one foot long
    meterstick, metrestick
    a rule one meter long (usually marked off in centimeters and millimeters)
    yard measure, yardstick
    a ruler or tape that is three feet long
    type of:
    measure, measuring rod, measuring stick
    measuring instrument having a sequence of marks at regular intervals; used as a reference in making measurements
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