Tao may not be defined, although people are always trying to define it.
The dictionary gives it a shot:
...the course of life and its relation to eternal truth.
...that in virtue of which all things happen or exist.
And here's a very interesting definition:
...it is the open road of escape from the solitary confinement of individuality.
Taoism is easier to define, since it describes what certain people do:
The dictionary, again:
...philosophical system developed by Lao-tzu and Chuang-tzu advocating a simple honest life and noninterference with the course of natural events.
And according to a crow:
Tao: That which one did not realize one was part of.
Taoism: The one thing that one should never do.
Taoist: One who still thinks in terms of labels.