Khan was the name given to male rulers of the Mongols, Tartars, or Turkic/Altaic peoples of Central Asia, with female rulers called khatun or khanum. Though the term seems to have originated with the Turkic peoples of the high inner steppes, it spread to Pakistan, India, Afghanistan and Persia through the expansion of the Mongols and other tribes.
Many of the great Silk Road oasis towns were ruled by khans during their heyday, but so were great city-states of the Mongol and Turkic empires of their age, and the rise and fall of khans subsequently have greatly shaped the history of Central, Southeast and Eastern Asia - from the brief and violent Mongol khans to the modern rulers of Turkey.
Different Rulers, Same Name
The first known use of the word "khan," meaning ruler, came in the form of the word "khagan," used by the Rourans to describe their emperors in 4th to 6th century China. The Ashina, consequently, brought this usage across Asia throughout their nomadic conquests. By the middle of the sixth century, Iranians had written reference to a certain ruler called "Kagan," the king of the Turks. The title spread to Bulgaria in Europe around the same time where kans ruled from the 7th to 9th centuries.
However, it wasn't until the great Mongol leader Genghis Khan formed the Mongol Empire - a vast khanate spanning much of South Asia from 1206 to 1368 - that the term was made popular to define rulers of vast empires. The Mongol Empire went on to be the largest land mass controlled by a single empire, and Ghengis called himself and all his successors the Khagan, meaning "Khan of Khans."
This term carried over to different spellings, including the name Ming Chinese emperors gave their minor rulers and great warriors, "Xan." The Jerchuns, who later founded the Qing dynasty, also used the term to denote their rulers.
In Central Asia, the Kazakhs were ruled by khans from its founding in 1465 through its breaking into three khanates in 1718, and along with modern-day Uzbekistan, theses khanates fell to Russian invasion during the Great Game and its subsequent wars in 1847.
Still today, the word khan is used to describe military and political leaders in the Middle East, South and Central Asia, Eastern Europe and Turkey, especially in Muslim-dominated countries. Among them, Armenia has a modern form of khanate along with its neighboring countries.
However, in all of these cases, the countries of origin are the only people who might refer to their rulers as khans - the rest of the world giving them westernized titles like emperor, tsar or king.
Interestingly, the main villain in the hit franchise series of films, comics books "Star Trek," Khan is one of the main super-soldier villain and arch-nemesis of Captain Kirk.