The Collaborative International Dictionary of English v.0.48:

yin and yang \yin and yang\, n. (Chinese philosophy)
   the two fundamental principles, one negative, dark, passive,
   cold, wet, and feminine (yin) and the other (yang) positive,
   bright, active, dry, hot and masculine. The interactions and
   balance of these forces in people and nature influence their
   behavior and fate. --[RHUD]
   [PJC]
   [1913 Webster]