From my personal experience, either from my own track record or my clients’ my definition of success is this:
Decide on your goal and then be consistent in taking all the steps you need to realize it. This means, you keep focussed and never conform to external factors or people’s ideas that might stop your journey.
Your goal might be anything from getting that six-pack belly to getting a PHD.
Having said that, I would like to add that a GOAL should be something that makes you happy once realized, and/ or something that creates some goodness in the world, absolutely no harm.
Becoming a good parent, opening a small gift shop, running a marathon, getting rid of the extra weight you got from your vacation, finishing a book, acquiring an new skill, creating a vegetable garden, becoming a volunteer, climbing mount Everest, travelling around the world with a back pack, becoming a millionaire, becoming the CEO of a great firm, giving TED Talk.
Success is doing whatever it takes, consistently, until you realize your goal. And goals have no limit or measure.