A career-defining moment or action is one that is very important in making someone successful in their job or in deciding how they will be remembered.

The movie features Bruce Willis in his career-defining role. 布鲁斯·威利斯参加了这部电影的演出，成为他演艺事业成功的关键时刻。

a career-defining speech/performance/victory 决定事业成功的演讲／演出／胜利