Autonomy is one of the words that some unschoolers like to use but others aren't happy with. A lot of definitions of autonomy use it synonymously with independence, which I find problematic – unschooling isn’t about encouraging kids to be independent per se. I came across a definition of autonomy that I have found helpful, in terms of radical unschooling, on Wikipedia: the capacity of a rational individual to make an informed, un-coerced decision.
What I find useful about that particular definition is it opens up a can of worms! How rational are children? How is it possible for children to be informed? And to what extent can a child really act on any given decision?