What is Zhouyi?

The Zhou Changes — the core text of the I-Ching before the Ten Wings commentaries were appended. Contains the 64 hexagram judgments (卦辭) and 384 line texts (爻辭). The name refers to the Zhou dynasty, under whose early kings the text took its received form.

Source: Zhouyi Zhushu (周易注疏), Wang Bi & Kong Yingda