Wikipedia
Dayuan (Ta-yuan; Old Chinese reconstructed pronunciation: /daːds qon/; Middle Chinese reconstructed pronunciation according to Edwin G. Pulleyblank: /daj ʔuan/; ) was a country in Ferghana valley in Central Asia, described in the Chinese historical works of Records of the Grand Historian and the Book of Han. It is mentioned in the accounts of the famous Chinese explorer Zhang Qian in 130 BCE and the numerous embassies that followed him into Central Asia. The country of Dayuan is generally accepted as relating to the Ferghana Valley.
These Chinese accounts describe the Dayuan as urbanized dwellers with Caucasian features, living in walled cities and having "customs identical to those of the Greco-Bactrians", a Hellenistic kingdom that was ruling Bactria at that time in today’s northern Afghanistan. The Dayuan are also described as manufacturers and great lovers of wine.
The Dayuan were probably the descendants of the Greek colonists that were settled by Alexander the Great in Ferghana in 329 BCE (see Alexandria Eschate), and prospered within the Hellenistic realm of the Seleucids and Greco-Bactrians, until they were isolated by the migrations of the Yuezhi around 160 BCE. It appears that the name "Yuan" was simply a transliteration of Pali Yona or Sanskrit Yavana, used throughout antiquity in Asia to designate Greeks (“ Ionians”), so that Dayuan would mean "Great Ionians".
The interaction between the Dayuan and the Chinese is historically crucial, since it represents one of the first major contacts between an urbanized Western civilization and the Chinese civilization, opening the way to the formation of the Silk Road that was to link the East and the West in material and cultural exchange from the 1st century BCE to the 15th century.