Zhou's key terms Wuji and Taiji appear in the famous opening phrase wuji er taiji 無極而太極, which Adler notes could also be translated "The Supreme Polarity that is Non-Polar!"
Robinet explains the relationship:
Non-polar (wuji) and yet Supreme Polarity (taiji)! The Supreme Polarity in activity generates yang; yet at the limit of activity it is still. In stillness it generates yin; yet at the limit of stillness it is also active. Activity and stillness alternate; each is the basis of the other. In distinguishing yin and yang, the Two Modes are thereby established. The alternation and combination of yang and yin generate water, fire, wood, metal, and earth. With these five [phases of] qi harmoniously arranged, the Four Seasons proceed through them. The Five Phases are simply yin and yang; yin and yang are simply the Supreme Polarity; the Supreme Polarity is fundamentally Non-polar. [Yet] in the generation of the Five Phases, each one has its nature. (tr. Adler 1999: 673-74)
The taiji is the One that contains Yin and Yang, or the Three (as stated in Hanshu 21A). This Three is, in Taoist terms, the One (Yang) plus the Two (Yin), or the Three that gives life to all beings (Daode jing 42), the One that virtually contains the multiplicity. Thus, the wuji is a limitless void, whereas the taiji is a limit in the sense that it is the beginning and the end of the world, a turning point. The wuji is the mechanism of both movement and quiescence; it is situated before the differentiation between movement and quiescence, metaphorically located in the space-time between the kun 坤, or pure Yin, and fu 復, the return of the Yang. In other terms, while the Taoists state that taiji is metaphysically preceded by wuji, which is the Dao, the Neo-Confucians says that the taiji is the Dao. (Isabelle Robinet, "Wuji and Taiji 無極 • 太極 Ultimateless and Great Ultimate," in The Encyclopedia of Taoism, ed. Fabrizio Pregadio [Routledge, 2008] 1058)