In addition to the definitions mentioned below, Othello also fits many other conventions of Aristotle's definitions of tragedy that Shakespeare used and adapted.
Aristotle calls for a tragic hero to have or experience some sort of harmatia. Most commonly, this word is translated as a "flaw" or "mistake." Othello fits this definition because he makes the mistake of falling for Iago's tricks. He shows his flaws by becoming increasingly jealous and suspicious, and at the end admits that he is someone who "loved not wisely, but too well" (5.2.404).
Aristotle also says that the tragic hero should go through anagnorisis (a recognition or "knowing again") and peripeteia (a reversal). At the end of the play, Othello realizes the terrible mistake that he's made. He sees that he has been trusting Iago all along and thinking that Iago was honest and that his wife was false, but in reality Iago has been deceiving him while Desdemona has been loyal and faithful. Othello sees how incorrect he has been about the entire situation. He goes through a complete reversal of emotions and fortune. He was once respected and happily married, but has destroyed his own happiness because of his jealousy and anxiety.
Additionally, Aristotle's definition calls for catharsis. Catharsis is a release or a purification of the feelings of pity and fear that have been built up over the play. At the end of Othello, although the ending is sad, the audience should have a feeling of resolution. Iago is exposed and imprisoned, and Othello kills himself out of remorse.