The I Ching (Wade-Giles) or "Yì Jing" (pinyin), also known as the Classic of Changes, Book of Changes and Zhouyi, is one of the oldest of the Chinese classic texts. The book contains a divination system comparable to Western geomancy or the West African Ifá system; in Western cultures and modern East Asia, it is still widely used for this purpose.
During the Warring States Period, the text was re-interpreted as a system of cosmology and philosophy that subsequently became intrinsic to Chinese culture. It centred on the ideas of the dynamic balance of opposites, the evolution of events as a process, and acceptance of the inevitability of change.
Source: Wikipedia, "I Ching", available under the CC-BY-SA License.