Khan
Khan is a title meaning ruler in Mongolian. A khan controls a khanate. Whenever appropriate, it is also translated as emperor.Probably the most famous people with the title Khan were the Mongol Genghis Khan, and his grandson Kublai Khan who founded the Mongolian dynasty in China, known as Yuan Dynasty.
This title was probably first used by Rouran; maybe the Xianbei used it earlier. At that time it was pronounced Khaghan. The gh sound later weakened and disappeared becoming Khaan in Modern Mongolian. The Secret History of the Mongols clearly distinguishes Khaghan and Khan: only Genghis and his descendants are called Khaghan ("Khan of Khans"); other rulers are referred to as Khan.
The title Khan was also used in the Seljuq Islamic dynasties of the near-east to designate a head of multiple tribes, clans or nations, who was below an Atabeg in rank.
Jurchen and Manchu rulers also used the title Khan (Han in Manchu); for example, Nurhaci was called Genggiyen Han.
Rulers of Khazaria used the title Khagan.
See also: Khong Tayiji, list of Mongol Khad
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In Star Trek, the last man to take the title Khan is Khan Noonien Singh, a genetically engineered madman who launches the Eugenics Wars in 1992 and battles Captain Kirk's Enterprise. He is eventually defeated by Kirk.