The term first described the original state of everything, which is profound and mysterious. Laozi used it to describe Dao and virtue as being in a profound and mysterious state, calling Dao “a mystery within a mystery,” or “virtue of mystery.” Ancient Chinese thinkers like Yang Xiong and Ge Hong went a step further, describing xuan (玄) as being the supreme original source or the primal ontological existence of all things in heaven and on earth. In this sense xuan is a kind of absolute existence, formless and imageless, which transcends all concrete things. Later on, xuanxue (玄学), or learning of the mystery, developed, referring to the quest into the original source or ontological existence of the world.
You and wu are from the same origin but have different names. They are all extremely mysterious and profound and lead to all changes. (Yang Xiong: Supreme Mystery)
Xuan is the origin of nature and the source of all things. (Ge Hong: Baopuzi)