Wiktionary
defining moment
n. 1 A moment in time that defines something, such as a person's success or failure, achievement or demise, talent, etc. 2 A point at which the essential nature or character of a person, group, etc., is revealed or identified.
Usage examples of "defining moment".
It was another life-defining moment, less pleasant than Ben and Kate’.
He wanted to know more about the shooting that put him in the chair, that being a defining moment in Mike's life.
It would be a shame if you were to miss what is likely to be the defining moment of your life's experience.
Her death in that transporter mishap was the defining moment that taught him the true impermanence of physical existence….
He stood before the king in what should have been the defining moment of his life.