One of the hardest things to define. Here is how Edward de Bono tries to define it:
At no point in this book am I going to define ‘success’. We all have an idea of what the term means but a precise definition is impossible. (There is exactly the same trouble with the world ‘creativity’)
Is it success in the eyes of the world: winning an Olympic gold medal or at Wimbledon; making a great deal of money; running a large organization; making things happen; being awarded the Nobel Price?
Or, is it personal success: the person who toward the end of his or her life feels that it has been a happy, fulfilled, and enjoyed life?
Which is the more successful, a man who has made millions but is unhappy and unsatisfied or an unnoticed person who has led a happy life?
There are clearly many different ways of looking at success and it is only the poverty of the English language which provides us with but a single word for them.
Perhaps the simplest definition is ‘to set out to do something and to succeed in doing it‘.
(Tactics, The Art & Science of Success, Page 9)