Yin-Yang symbol. This ancient but intriguing symbol represents many related ideas. It is seen especially in Asia, where it has been adopted by Taoism and other religious philosophies. Generally the Yin-Yang is meant to represent the balance of forces of nature, the give-and-take dynamic that creates harmony, stability, and peace. Note the small dots of opposite color in the larger units. These are usually added to indicate that each unit contains a small amount of the other; they are not pure opposites.
Complementarity in ideas is a Western way of expressing the same counterbalance. But the Yin-Yang is a more concise way of saying this.