Western Regions

The Western Regions or Xiyu () refers to Central Asia, specifically west of the Yumen pass. It was used in Chinese between the 3rd century BC to the 8th century AD. 
It sometimes refers to Xinjiang or even northern parts of the Indian subcontinent (as in the novel "Journey to the West").
Because it is located on the Silk Road, the xiyu have been important since at least the 3rd century BC.
History.
The Han–Xiongnu War (ended 89 AD) was fought there. In the 7th century, the Tang campaign against the Western Regions led to Chinese control of the region. 
The xiyu became an important place of exchange between East Asia, the Indian subcontinent, and the Muslim world.
This was especially true during the Mongol Empire. Buddhism spread through the region to China. 
The Tang dynasty monk Xuanzang crossed the region on his way to study in India. When he returned to Chang'an, the influential "Great Tang Records on the Western Regions" was written.