What is the defining moment of The Absolutely True Diary of a Part-Time Indian?
In any coming of age work, there are many defining moments because the compendium of the work based on a life consists of "defining moments." Those are the ones that entered into the work. This means that a case can be made for any moment featured in the novel as representative of one that is definitive.
I think that the rematch between Reardon and Wellpinit is probably my vote for the defining moment. Junior and Reardon entered the game on a 12 game winning streak, with their sole loss at Wellipnit. The loss was marred by a hard foul on Junior by Rowdy, as well as animosity from the crowd aimed at Junior. At the same time, Junior himself had overcome the difficulty of transitioning to Reardon, telling his Social Studies teacher who is insensitive to his predicament his own conception of the world:
...the world is only broken into two tribes...people who are assholes and...people who are not.
With this realization in hand and buoyed by an increased confidence, Junior plays the game of his life against the Reservation school and against Rowdy. He vows to never acquiesce to anyone and the game becomes the forum where this mantra is put to its first and ultimate test. He succeeds and Reardon slaughters Wellpinit. During the celebration, Junior sees the inescapable sadness that seems to follow the Native American players even away from the reservation. He understands the differences between both worlds and in the midst of his greatest happiness, he retreats to the locker room and cries. His tears are representative of how he has a foot in both worlds, and how the dual consciousness he experiences in both will forever help to form his identity and always be a part of him.