The simplest modern definition of wuji is the state before, or more precisely the state before creation. The problem with this definition is that it implies a definition of creation, and defining creation is a touchy subject.
The historical and more literal definition of wuji is without ridgepole. A ridgepole is a timber laid along the ridge of a roof. Attaching the upper ends of the rafters to this pole creates a sloping roof. Adding a ridgepole to a structure creates a horizontal apex that separates the roof into two halves. This separation is taiji.
Laozi and Zhuangzi were the first to use wuji in a cosmological sense. These daoist philosophers said that by following the Dao the true sage can return to the state of infinity or without pole.
When discussing the martial art of Taijiquan, the concepts of wuji, taiji, yin and yang, and the five phases are illustrated with the Taijitu diagram and its explanation as presented by the Neo-Confucian scholar Zhou Dunyi in his Explanation of the Diagram of the Supreme Ultimate. In this work Zhou links the states of wuji and taiji, yin and yang, the five phases, and seasonal changes into one cohesive system that has become a seminal reference.
"Wuji and yet taiji. Taiji in generating 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 taiji, taiji is fundamentally wuji."