The Dao 道
The text uses Pinyin, the current intenational standard. The older Wade-Giles which is still used in in parentheses.
The Dao (or Tao) is The Way or more concretely the path. The Daodejing (Tao te Ching), written in around 530 BC, attributed to Laozi (Lao Tzu) is considered the founding text for Daoism as a philosophy. The first verso of the Daodejing (Using Legge's and Muller's translation):
The Dao that can be trodden
is not the enduring and unchanging Dao.
The name that can be named
is not the enduring and unchanging name.
If we perceive it as having no name,
it is the Originator of heaven and earth;
If we give it a name,
it is the Mother of all things.
Always without desire
we see the mystery,
But we desire,
Only the outer fringe we shall see.
These two aspects, really the same;
though we give it different names.
This sameness is the mystery,
A mystery within mystery;
The door to all marvels.
It is mysterious, but still, at least until we examine it. Then it becomes the Mother of all things. As the idea of Yin and Yang developed, it becomes the Mother of all things changing, in a Yin-Yang cycle, each according to its nature.
As the Yin-Yang of nature was observed and the Yin-Yang exhibited as a life-force, this was called Qi (Chi, Ch'i).