yin and yang
Yin and yang are fundamental concepts in Chinese philosophy and medicine. Yang embodies positive, active, "male" qualities and thus compliments yin, which embodies negative, passive, "female" qualities. The concepts of yin and yang are also central to the theoretical basis of traditional Chinese medicine and macrobiotics.
In Taoism, the body and the universe as a whole can be understood in terms of these two distinct but complementary principles. The one can't exist without the other, so that nothing is ever completely yin or yang, but a combination of the two.