Yin and yang (Traditional Chinese: 陰陽; Simplified Chinese: 阴阳; Hanyu Pinyin: yīnyáng; Korean hangul: 음양; hanja: 陰陽; revised: eumyang; McCune-Reischauer: ŭmyang; Vietnamese: Âm-Dương) originates in ancient Chinese philosophy and metaphysics, which describes two primal opposing but complementary forces found in all things in the universe. Yin, the darker element, is passive, dark, feminine, downward-seeking, and corresponds to the night; yang, the brighter element, is active, light, masculine, upward-seeking and corresponds to the day.
Source: http://en.wikipedia.org/wiki/Ying_Yang

"Yin & yang are descriptions of complementary opposites rather than absolutes" I like that idea ~