There are lots of definitions that stem from John Holt's coined term of "unschooling". I'll paste one below. For more information, see
The Teenage Liberation Handbook
www.sandradodd.com/unschooling
www.unschooling.com
Mary Griffith's definition
Mary Griffith is the author of The Unschooling Handbook
What is Unschooling? from the Texas Unschoolers site
From a longer Q&A article on getting into college by Alison McKee, here is Alison's clarification on what unschooling is and isn't:
Q: What is unschooling?
A: Unschooling is a term that the late John Holt coined in the late ‘70's to describe learning that is based on a child's interests and needs. Unschooling does not begin with a parent's notion of what is important to learn and then turn the choices of how to learn the content over to a child. Rather, it begins with the child's natural curiosity and expands from there. Unschooling is not "instruction free" learning. If a child wants to learn to read, an unschooling parent may offer instruction by providing help with decoding, reading to the child, and giving the child ample opportunity to encounter words. If the child is uninterested in these supports, the parent backs off until the child asks for help. The most important thing about the unschooling process is that the child is in charge of the learning, not the adult. Unschoolers often do no traditional school work, yet they do learn traditional subject matter. They learn it as a natural extension of exploring their own personal interests.

