Yang Jian
The first recorded use of the term "Türk" or "Türük" as an autonym is contained in the Old Turkic inscriptions of the Göktürks ("Celestial Turks") of Central Asia (c. AD 735). The Turkic self-designation "Türk" is attested to reference to the Göktürks in the 6th century AD. A letter by Ishbara Qaghan to Emperor Wen of Sui in 585 described him as "the Great Turk Khan."