Dao

The dao that can be described is not the Eternal Dao;
the name that can be named is not the Eternal Name.

Nameless is the origin of Heaven and Earth;
naming is the mother of the Myriad Things.

Therefore,
Completely without attachments, you see the wonders;
completely attached, you see the outward forms.

These two
are same in origins, but different in names.
This sameness is the mystery,
the mystery of all mysteries,
a door to all wonders.

-- Dao De Jing (chapter 1)

Dao (Tao) is one of the names of the Ultimate Principle.