Personally, my definition of success is when I'm content with my career and where I am and who I am in my life, generally. I don't aspire to be the greatest among the best, but rather live my life to the fullest. When I can make myself proud by looking back at what I've done in my life, that's when I'm truly successful.
We sometimes define success as having lots of money, being the boss at your work, etc. but success could also be being happy with what you have right now.