Taixu (Great Void)
Taixu (太虚) refers to a state of void in both space and things. Zhang Zai, a thinker in the Song Dynasty, elaborated on the meaning of taixu, or great void. He believed that all things in heaven and on earth were made up of qi (气), and that taixu was its natural state, which was formless and motionless. When taixu coalesced, it turned into qi; when qi dissipated, it became taixu. Though taixu could not be felt by humans, it was not absolute emptiness and nothingness. Taixu gave life to all things in heaven and on earth by means of qi.
And so, they will not be able to go beyond Mount Kunlun, nor can they wander in the great void. (Zhuangzi)
Taixu is formless; it is the original state of qi. Whether qi coalesces or dissipates, it is just a temporary form of taixu, or great void. (Zhang Zai: Enlightenment Through Confucian Teachings)