What is "Tao"? What is the core concept of Taoism?

The Tao is the origin and fundamental law of the universe. Laozi begins the Tao Te Ching: "The Tao that can be told is not the eternal Tao; the name that can be named is not the eternal name." That which can be expressed in language is not the unchanging, eternal Tao. Tao is the ultimate reality beyond language and concepts.

Tao and Te: Tao is the essence, Te is its manifestation. Tao is the fundamental law of all things, while Te is how Tao manifests in individual life.

The "Tao" in Taoism has three meanings: 1) The Tao as cosmic origin — from which all things arise; 2) The Tao as natural law — the fundamental principle of operation; 3) The Tao as spiritual path — the supreme state attained through cultivation. The everyday word "tao" (way/method) is a much narrower usage.