Concept
In the Chinese language, Yin 
means the shady side and Yang 
the sunny side of a hill. By decomposition of Chinese characters, the left sides of the words 
and 
mean the hill and the right sides mean the moon and the sun, respectively. In traditional Chinese medicine, “Yin and Yang are the law of heaven and earth, the principle guiding analysis of natural and social changes, the origin of all changes, and the root of birth and destruction, … essence in treatment of disease”
Yin–Yang ( – ) theory is one of the three basic theories in traditional Chinese medicine (TCM). It is a philosophical conceptual framework representing a radically different logic than the thought process of Western philosophical thinking, which holds that a pair of contraries cannot both be true. Contrary to this idea, the Yin–Yang concept believes that Yin and Yang represent opposite yet interdependent qualities of everything or phenomena in the universe. In other words, each thing or phenomenon could be itself and its contrary. Further, Yang contains the seed of Yin, so Yang can transform into Yin and vice versa.
In ancient Chinese philosophical terms, Yin and Yang represent opposite but complementary qualities. The two qualities of an object or phenomenon are opposite yet interdependent. Every phenomenon in the universe has two opposites, Yin and Yang, which are both in conflict and mutually dependent. This is the universal Law of Unity of Opposites. The nature of Yin–Yang and their relationships represent a view and analysis of natural and social interaction. The essential principle of Yin–Yang theory is that any object or phenomenon in the universe is composed of two interdependent parts, Yin and Yang, the balance and opposition of which are dynamic. As such, the Yin–Yang concept is a thought process and the way to analyze and interpret the natural and social phenomena and change. Yin is the material basis for the change, while Yang is the motive force and commander for the dynamic change.
The long developmental history of Yin–Yang theory is described in the Book of Changes (Yi Jing, 
, 700 B.C.), as broken and unbroken lines. During the Warring States Period (
, 476–221 B.C.), Yin–Yang theory was systematically developed and applied to traditional Chinese medicine.