Funny, a lot of people must be reading "Poetics". I just answered another posting about this very subject. So, to start off the discussion, I am going to paste the answer I posted earlier so as to give others the chance to offer their own views on the subject.
The term hamartia was introduced in Aristotle's Poetics. According to Aristotle, the term refers to mistakes, sins, to err, and wrongdoings. Aristotle used this term to define a wrong committed against another person.
Over time, like many words, the meaning of hamartia has changed. Many critics have stated that Aristotle used the term wrongly in his text and that the true meaning of the word was more geared toward the err one commits when he/she does not know any better.
Aristotle's use of the word tended to lean towards the tragic flaw of a person based upon their own limits or weaknesses. While similar to that of alternative views, Aristotle's meaning encompasses that the flaw does not concede to the fact that the one in error is not wholly innocent and one cannot find him less morally at fault.