Person, place, and
language. Chagatay (or Chaghatay, or Jagatai) was the second son of
Genghis Khan, on whose death he received an inheritance in
Central Asia, including
Turkestan and
Transoxiana, and which he later expanded into
Afghanistan. He was a respected lawman, the most adept of the sons in his father's
laws. He died in about 1241.
This Central Asian empire was thereafter known as Chagatay Ulus. It was divided in two, western and eastern, in 1347; reunited in 1360; but in 1363 fell under the rule of Timur (Tamerlane) and Sultan Husayn, with Timur sole ruler from 1369/1370. The Turkic nomads of the empire were also known as Chagatay (as opposed to Moghul in the east of the empire), and Byzantine sources also refer to the people as Tzakhatai or variants.
Their Turkic language, most closely related to the modern Uzbek and Uyghur (Uigur) languages, continued to be the literary and diplomatic language of all the Turkic successor states, such as Khiva and Bukhara, up to the twentieth century. Only in 1921 did Uzbek officially replace it in Uzbekistan.
It was shaped as a classical language especially by Nava'i (1441-1501), who imported the elegant stylistics of classical Persian. The classical period of Chagatay went from 1465 to about 1600, beginning with the publication of Nava'i's first divan.
It was written in Arabic script, or occasionally in the Uyghur script, neither of which were suitable for it, so the exact pronounciation may in some cases be dubious. But it is so close to modern languages such as Turkish that there is no serious confusion.
Thanks to mirv for correction on accession of Timur.