Yin and yang originally meant the shady and sunny sides of a hill. They became the two complementary aspects of all phenomena: yin dark, soft, receptive and still; yang bright, firm, active and moving. Each contains the seed of the other, and their alternation drives the cycles of nature.
Yin and yang
The complementary forces of Chinese thought: dark and light, receptive and active, whose interplay makes all change.
Also: yinyang