How is ignorance defined at the end of The Adventures of Huckleberry Finn?
What do you mean by ignorance precisely? The reason is that ignorance is not mentioned directly in the final chapter of this novel. What the novel does end with though is the determination of Huck that he will not be civilised as a result of Aunt Sally's ministrations:
But I reckon I got to light out for the Territory ahead of the rest, because Aunt Sally she's going to adopt me and sivilise me, and I can't stand it. I been there before.
In a sense, the end of this novel refers to a kind of ignorance in two senses. First of all, there is the ignorance of Aunt Sally in failing to understand that now, after all of his adventures, Huck Finn is a character that is never going to be "sivilised" according to the narrow expectations of society. Secondly, we could argue that there is an ignorance on the part of Huck as he deliberately shuns society and his chance to have a place in it. This last example shows that ignorance is something that he knowingly desires as he wants a life where he is free and unrestricted to do what he wants, and given the bad examples of civilisation he has seen during his journeying, it is no wonder.