Khan (sometimes spelled as xan, han, Polishchan) is a title meaning ruler in Mongolian and Turkish.
Political Khans
A khan controls a khanate (sometimes spelled chanat). Whenever appropriate, it is also translated as king.
For the higher, rather imperial title Khagan ("Khan of Khans") , see that article - it applies to probably the most famous people with the title Khan: the MongolGenghis Khan, and his grandson Kublai Khan: the former founded the Mongol Empire and the latter founded the Yuan Dynasty in China. The ruling descendents of Genghis Khan are referred to as the Great Khans.
The title Khan was among numerous titles used by the sultans of the Ottoman Empire as well as the rulers of the Golden Horde and its descendant states. The title Khan was also used in the Seljuk Turk 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 the Gokturks, Avars and Khazars used the higher title Kaghan, as rulers of distinct nations.
The title Khan was not uncommon in some of the polities of the various -generally animistic or islamic- people called Ta(r)tars (an anachronistic reference to tartaros, the classical Greek hell, since Genghis Khan's conquering, ransacking Mongol hordes terrorized christianity without precedent, as if the apocalypse had started) in territories of the Mongol Golden Horde and its successor states, all in time to be subdued by Muscovia which became the Russian Empire, including Sibir Khanate (giving its name to Siberia as the first significant conquest during Russia's great eastern expansion across the Ural range), Khanate of Kazan, Astrakhan Khanate, Nogai Khanate and Crimean Khanate. An example of a humbler dynasty of Tartar khans is the Qasim Khanate (hence modern Kasimov), named after its founder, a vassal of Moscovia/Russia.
Kings of Silla, an ancient kingdom of Korea, were titled as Marib-Khan which means the head of kings. For example, King Naemul was called Naemul Marib-Khan.
Khan was the title used by Bulgar rulers. The greatest Bulgar clan was the "Dulo". The Bulgar khans came to prominence in the fourth and fifth centuries AD, when the they came to Europe. Among the best known Bulgar khans were Khan Kubrat - founder of Great Bulgaria, Khan Asparukh - founder of Danubian Bulgaria (today's Bulgaria), Khan Tervel, "The savior of Europe", Khan Krum "The Terrible". "Khan" was the official title of the ruler until 864 AD, when Kniaz Boris (known also as Tsar Boris I) adopted the orthodox faith.
Adil Khan
Military ranks
The title khan was also used as a military officer rank in certain armies, especially following the decimal organisation (already known from Achaemenid Persia) of Genghis Khan's conquering 'hords' (actually under the strictest discipline). In some Muslim-states in India, especially the Delhi sultanate (which absorbed all under the Mughal dynasty), a Khan was a high-ranking imperial general, commanding at least a lac (i.e. 100.000) horseman, being placed over 10 Maliks, each of which commanded Amirs (so 100 under a Khan), who each command 10 Sipah salars (so 1000 per Khan), each commanding 10 Sar-i-khails (so 10.000 per Khan) who each commanded 10 ordinary horsemen - thus it was described in person by Sultan Nasiruddin Mahmud bughra khan advising his son Muizuddin Kaiqobad (sultan of Delhi, 1287-1290).
Honorary titles
The titles Khan and Khan Bahadur (one rank higher) were also bestowed in India by the Great mughal, and later by the British Raj, as an honor akin to the ranks of nobility, often for loyalty to the crown.
It seems unclear whether the series of titles known from the Bengal sultanate, including Khan, Khan ul Muazzam, Khan-ul-Azam, Khan-ul-Azam-ul-Muazzam etc and Khaqan, Khaqan-ul-Muazzam, Khaqan-ul-Azam, Khaqan-ul-Azam-ul-Muazzam etc, are merely honorific or perhaps relate to a military hierarchy
Often used a family name, Khan can have one of several connotations, all related to some extent to the same root as described above. The term is used almost universally as a surname, or almost a suffix by people of Pashthoon (Pathans) ethnicity living in Afghanistan, India, Bangladesh and bordering areas of Pakistan. In this way, it is often a synonym for Pashthoon/Pathan. It is also a family name for people that were bestowed the title of Khan or Khan Bahadur by the British Raj.
Fiction
In the BattleTech universe, each Clan has two leaders named Khans, and during war, all Khans are directed by an ilKhan.