As a coach I ask that question a lot. Early in the coaching relationship as my client and I are setting goals for the coaching journey the word success invariably comes up. (Side note, when I Googled "I want to be successful" I got 1.120 billion results)
And asking what success means to the client is where the seemingly infinite definitions appear. Some clients long term goal is "to be successful". When I ask for a definition? "I want to be rich", "I want to have a big house", "I want to be happy", "I want to be <insert desired job title here>". Some clients define success by being "as successful as my colleagues", yet others tell me, "I want to make a difference in the lives of others".
But whatever the answer, knowing your definition of success, is a crucial part of coaching. Crucial because knowing what the goal looks like helps you know when you are making progress, helps you know when you've arrived.