Daoist vocabulary describing a stage in Daoist creation theory which sits between wuji and yin yang. In its modern context, taiji is often recognized as “tai chi”, which is popularly considered to cover a range of arts as diverse as qigong and taijiquan.
Daoist theory relies much on scalability, so that many theories operate at a macrocosmic and microcosmic level. That is to say that what happens on an externalised, global, or even universal scale can also happen at an internalised level even to atomic scales within humans. Therefore the principle of taiji – what physicists describe as angular momentum – is considered a primary function in the development of physical and internal practices .
For more about Daoist creation theory, see Steve Galloway’s essay, The Nature of Taijiquan.