The definitive moment in the play A Doll's House occurs when Nora comes to the realization that, throughout her life, she has been over exerting herself in aims to help and entertain those whom she loves.
[Nora's father] called me his doll-child, and he played with me just as I used to play with my dolls.
She also understands that her marriage to Helmer has been a one-sided relationship where, again, she serves more as a conversation piece rather than as wife and mother.
I have existed merely to perform tricks for you, Torvald. But you would have it so. You and papa have committed a great sin against me. It is your fault that I have made nothing of my life.
It could be argued that a definitive moment may also be that Helmer finally shows his true colors after reading Krogstad's letter, and that this is the agent of change that moves Nora to reconsider her role, not only within the household, but in her life as a whole.
While this is admissible as an argument, it is not "the" definitive moment. It certainly makes for quite a pivotal moment do to how it influences the main character, but it is the actual realization of Nora's own tragic flaw what determines whether the character has come full circle. It is clear that Nora grows up at the very end, to the point of radically changing who and how she is.