yin and yang

According to traditional Chinese philosophy, yin and yang are the two primal cosmic principles of the universe. Yin is the passive, female principle. Yang is the active, masculine principle. According to legend, the Chinese emperor Fu Hsi claimed that the best state for everything in the universe is a state of harmony represented by a balance of yin and yang. Unsurprisingly, legend has it that, according to Fu Hsi, true harmony requires yang to be dominant. It's just the nature of things.

In classical Chinese, yin and yang refer to the shady and sunny sides of a hill or valley.

Found at Skeptic.com