Yin-yang polarity is at the core of TCM. The terms yin and yang are impossible to translate. Originally yang meant the sunny side of a slope or the north bank of a river, while yin meant the shady side of a slope or the south bank of a river. These meanings were later extended to cover a vast range of polarities, so that, for example, yang came to refer to heat, movement, vigour, increase and upward or outward movement, while yin referred to cold, rest, passivity, decrease, and inward and downward movement. On the biological level yang is male, yin female.
It is essential to realize that although yin and yang are polar opposites they are not mutually exclusive. Yin always contains at least a trace of yang and vice versa. In the traditional yin-yang diagram this is indicated by the fact that yang contains a small spot of yin and yin a small spot of yang.
Perhaps the nearest Western equivalents would be the concepts of positive and negative in electricity and north and south in magnetism. But yin and yang are not thought of as static fixed entities; they constantly interact with each other and transform themselves into each other. In the whole of nature, as well as in ourselves, there is an ever-changing flow of yang into yin and yin into yang.
Our state of health is thought to depend on the balance between yin and yang. If either preponderates more than it should the result may be disease, which is thus thought of as resulting from a dynamic imbalance. (Underactivity or overactivity of endocrine glands, such as the thyroid, might be a modern instance of this principle.) Treatment is conceived of as a means of restoring the balance, and classical acupuncture is wholly concerned with this.