Tao Te Ching (TTC). (道德经 Dao de jing) The book does not specifically define what the Tao is, as a matter of principle. Fundamentally, Tao is undefinable, unlimited, and unnamable.
There was something undefined and complete, existing before Heaven and Earth. How still it was, how formless, standing alone and undergoing no change, reaching everywhere with no danger of being exhausted. It may be regarded as the mother of all things. Truthfully it has no name, but I call it Tao (TTC, chapter 25)
However, there are characteristics of Tao that are commonly noted and used to describe its functioning, particularly as guidelines for practicing te.
Tao is undifferentiated
All distinctions are actually relative comparisons bound together by their mutual reference. Thus (chapter 2) there is no such thing as 'long' except by comparison to 'short' and vice-versa; there is no such thing as 'being' except by comparison to 'non-being'. Because Tao itself has no shape or size, all comparisons fall within it, so there can never be 'real' differences. Often this is used to suggest a neutral, giving attitude - see TTC chapter 49.
"Return" is a complex concept: in one sense it is similar to 'nature abhors a vacuum' - "That with no substance enters there with no space" (TTC chapter 43); in another it reflects the natural cycles of the world (changing of the seasons, births of new generations); in yet a third it implies the natural return to quiescence that is the end result of all action (TTC chapter 14). This concept is often used to argue against forceful action, on the grounds that Tao (and its manifestations) will flow back, circumvent, and eventually undo any attempts to force it into a particular path.
Tao is subtle and quiet
The most important aspects of Tao are its unremarkable, unnoticed, everyday workings - "the softest thing in the world overcomes the hardest" (TTC chapter 43). Many places in the Tao Te Ching point out that dramatic, enticing or noteworthy events may catch the eye and assume significance, but that it is the slow, slight, unobserved and continuous movement of the manifestations of Tao that actually accomplish things. In this context, practitioners are cautioned to be unobtrusive, undemanding, and unsophisticated in their actions, and to know when to let go so that the unseen workings of Tao can carry the act to its completion.
Tao is simultaneously dispassionate and nurturing
Because all beings are manifestations of Tao, Tao - by definition - gives of itself wholly and completely to each. But by the same token, Tao is indifferent to the disposition of mere manifestations. Birth and death and life itself, from the perspective of Tao, are only movements and transformations of form. This is often used to suggest selflessness and detachment to practitioners; compare with the Buddhist notion of anatta (no-self).
In terms of western philosophy, the concept of Tao would be considered immanent, but it is a universal immanence that has no strict comparison to the normal (western) use of the term. There is nothing transcendent about Tao, no part of it that is separate from the universe.
In religious Taoism, Tao is understood in terms of these constituents: Jing 精 corresponding to energy; Qi 氣 or flow of energy; and Shen 神 or the Spirit. The triad Jing Qi Shen 精氣神 constitutes the Tao of all that is, and are represented as deities in the Three Pure Ones.