Success, as a word itself, is of course defined as accomplishing a goal.
But what I'm sure you mean is, how do you define a successful life?
If, a few years ago, someone had asked me what my ultimate goal in life was, I would have told them, "to be happy."
But lately I've realized that happiness is not enough for me. Happiness is merely a feeling, often brought on by the ego fulfilling its own goals. If all you want out of life is to be happy, that's easy. Listen to what society tells you and follow the path it wants you to follow. Feel free to indulge in entertainment for hours every day, living stagnantly for the rest of your life. You'll be happy when your favorite TV show comes on, or when your wife gives you a good time in bed. When you buy a nice sports car you'll be thrilled.
But at the end of the day, what's the point? No, happiness is not enough for me. I seek to become fulfilled. I seek to achieve. I seek to understand. When I'm old in my bed, about to push off, I don't want to think to myself, "what the hell happened?" I want to think to myself, "man, I really lived."