When asked to provide my personal definition for the undefinable Tao, I have my good days and bad days. There are times when I can rip off two or three somewhat cogent paragraphs. At other times, when I'm feeling less articulate, I struggle to put together even a few cogent words, let alone a whole sentence! When that happens, people ask me how I can a subscribe to a philosophy if I can't muster a definition of the main concept.
In the future, if I ever find myself tongue-tied, I think I could easily fall back on the following description from Eva Wong's The Shambhala Guide to Taoism:
Although the Tao is the source of all life, it is not a deity or spirit...In the Tao-te ching, the sky, the earth, rivers, and mountains are part of a larger and unified power, known as Tao, which is an impersonal and unnamed force behind the workings of the universe.About the only word in the foregoing description that I might change is force. In some ways, a force is still a specific something, a distinct entity of unknown size and parameters. The word I personally would replace it with is process. In my mind's eye, Tao is the name we give for the process of the workings of the universe.