To me, being successful means...
Finding out what fulfills you over the long term and then persevering through the obstacles and challenges that will present themselves to you. I say "over the long term" because no matter how much you love something, there will be days, weeks that are challenging.

My definition of success has changed over time. 
I thought success was a position, an award, an accolade or some other quantifiable thing. Now, I believe that kind of success also has a large element of luck as well. So therein lies the question, is a success that depends on luck really success at all? I'd argue not. Success is tied to your motivations, desires, effort, work, and resiliency. Therefore, success is more about finding a passion and driving forward through obstacles.