Yin-Yang

Taiji (The Supreme Ultimate)

Yang → Heaven · Active Motion · Light · Firmness · Generation  (The generative force, expansive energy)

Yin → Earth · Stillness · Darkness · Receptiveness · Completion(The containing force, condensed form)  

Taiji: The undifferentiated, primordial Qi of Yin and Yang not yet separated.
(As stated in the *Yizhuan (Commentaries on the Yi Jing)*: "In the Yi there is Taiji, which generates the Two Modes [Yin and Yang].")