Chagatai (چغتای, Čaġatāy), also known as Turki, Eastern Turkic, or Chagatai Turkic (Čaġatāy türkīsi), is a Turkic language that was once widely spoken across Central Asia. It remained the shared literary language in the region until the early 20th century. It was used across a wide geographic area including western or Russian Turkestan (i.e. parts of modern-day Uzbekistan, Turkmenistan, Kazakhstan, Kyrgyzstan), Eastern Turkestan (where a dialect, known as Kaşğar tılı, developed), Crimea, the Volga region (such as Tatarstan and Bashkortostan), etc. Chagatai is the ancestor of the Uzbek and Uyghur languages. Kazakh and Turkmen, which are not within the Karluk branch but are in the Kipchak and Oghuz branches of the Turkic languages respectively, were nonetheless heavily influenced by Chagatai for centuries.

Ali-Shir Nava'i was the greatest representative of Chagatai literature.

The word Chagatai relates to the Chagatai Khanate (1225–1680s), a descendant empire of the Mongol Empire left to Genghis Khan's second son, Chagatai Khan. Many of the Turkic peoples, who spoke this language claimed political descent from the Chagatai Khanate.

As part of the preparation for the 1924 establishment of the Soviet Republic of Uzbekistan, Chagatai was officially renamed "Old Uzbek", which Edward A. Allworth argued "badly distorted the literary history of the region" and was used to give authors such as Ali-Shir Nava'i an Uzbek identity. It was also referred to as "Turki" or "Sart" in Russian colonial sources. In China, it is sometimes called "ancient Uyghur".

In the twentieth century, the study of Chaghatay suffered from nationalist bias. In the former Chaghatay area, separate republics have been claiming Chaghatay as the ancestor of their own brand of Turkic. Thus, Old Uzbek, Old Uyghur, Old Tatar, Old Turkmen, and a Chaghatay-influenced layer in sixteenth-century Azerbaijanian have been studied separately from each other. There has been a tendency to disregard certain characteristics of Chaghatay itself, e.g. its complex syntax copied from Persian. Chagatai developed in the late 15th century. It belongs to the Karluk branch of the Turkic language family. It is descended from Middle Turkic, which served as a lingua franca in Central Asia, with a strong infusion of Arabic and Persian words and turns of phrase.

Mehmet Fuat Köprülü divides Chagatay into the following periods:

The first period is a transitional phase characterized by the retention of archaic forms; the second phase began with the publication of Ali-Shir Nava'i's first divan and is the highpoint of Chagatai literature, followed by the third phase, which is characterized by two bifurcating developments. One is preservation of the classical Chagatai language of Nava'i, the other the increasing influence of dialects of the local spoken languages.^{[citation needed]}

Uzbek and Uyghur, two modern languages descended from Chagatai, are the closest to it. Uzbeks regard Chagatai as the origin of their language and Chagatai literature as part of their heritage. In 1921 in Uzbekistan, then a part of the Soviet Union, Chagatai was initially intended to be the national and governmental language of the Uzbek SSR. However, when it became evident that the language was too archaic for that purpose, it was replaced by a new literary language based on a series of Uzbek dialects.