Such a simple question - yet probably as many answers as there are people. I rather like the definition of Spencer Johnson (of "Who Moved My Cheese" fame) who says success is becoming who you are capable of being and making progress towards worthwhile goals. However I'm also inclined to think success is about being at peace with yourself. The bottom line seems to be it's not what you've achieved but how you feel. My experience is that most people want to feel happy and at peace - but how to reach that state is another question.
I mentioned Byron Katie's The Work to you last year and having now read her book "Loving What Is" have a greater understanding of how much of our stress and unhappiness is caused by the way we argue with reality. We are always thinking things should be different to how they are..."John shouldn't talk to me like that"... "Susan should keep her bedroom tidy"... "The neighbours shouldn't make so much noise"... "I should be thinner, earn more, exercise more, work harder".
These thoughts are ways of wanting reality to be different than it is and as Katie says "When I argue with reality, I lose - but only 100% of the time. The only time we suffer is when we believe a thought that argues with what is. When the mind is perfectly clear, what is is what we want". What you think shouldn't have happened should have happened - because it did - and no amount of thinking in the world can change that. When we stop opposing reality we lose all the tension and frustration that we experience when fighting it.
The other thing we get stressed about is not just living our own business but living someone else's business as well. "You need to get a better job"..."You need to take care of yourself"..."I want you to be happy"..."You should stop smoking". How can we know what's best for anyone else - do we even know what's right for ourselves? Being in someone else's business is a recipe for stress and anxiety.
Katie says "A thought is harmless unless we believe it. It is not our thoughts, but the attachment to our thoughts, that causes suffering. Attaching to a thought means believing that it's true without inquiring. A belief is a thought that we've been attaching to, often for years." We then end up with a story that we convince ourselves is real.
So maybe success is stopping the argument with reality and minding your own business! For some more information on how to do that go to www.thework.org.