What it is
The Zhouyi (周易, "the changes of Zhou") is the heart of the I Ching: the 64 hexagrams, each with its name, its judgment and six line text entries, plus the two texts of use nine, use six. It doesn't include the Ten Wings, which came later. When both are read together as a classic, we call it the Yijing.
The name
Zhou is the dynasty that ruled China from the 11th century BCE. The Han commentator Zheng Xuan also suggested reading 周 as "all-encompassing": the changes that surround everything. Both readings live side by side.
Old sources speak of three books of changes: the Lianshan, the Guicang and the Zhouyi. Only the Zhouyi has come down to us whole.
How old it is
Tradition credits King Wen and the Duke of Zhou. Many scholars think it grew out of the records of court diviners and place it near the end of the Western Zhou, around the 9th and 8th centuries BCE. Its language gives it away: sacrifices, military campaigns, weddings, omens. The oldest known manuscripts date from the 4th century BCE onward, on bamboo strips and silk.
In a reading
Everything you read in the poetic layer of Cambios for a judgment or a line comes from the Zhouyi.