Taiji, literally the supreme ridgepole, enters philosophy through the Xici Zhuan, the Great Treatise appended to the Yijing (Warring States era): 'In change there is Taiji, which generates the two modes; the two modes generate the four images; the four images generate the eight trigrams.' One becomes two, four, eight, and ultimately the ten thousand things.
Popular usage collapses everything: the circle symbol, the exercise, the philosophy, all called tai chi. The fish diagram is actually the taijitu, a Song-dynasty diagram, while the park exercise is taijiquan, boxing named after the cosmology, not the reverse.
The metaphysical reading: Taiji is not a time before time but a standing option, the cosmos permanently containing its un-split state. Wuji er taiji, the limitless and then the supreme ultimate, describes not history but a depth available under every differentiated moment.
If everything descends from one division, what was the first thing that divided?