When self publishing didn't exist (or pre 2010 when it was considered mostly taboo) I defined success as being traditionally published. But prior to that I defined it as obtaining an agent.
After being small press published for awhile, I defined success as that publisher allowing me to continue writing the series I was currently involved in. And to do that I needed to keep my sales numbers up or my series would get dropped.
Now I mostly exclusively self publish and I define success in a different way: by readers. How many readers can I reach? Connect with? Connecting equals a following and that to me, in my own personal writing journey, is my definition of success. The money comes from that. See, I can't do anything to control the money coming in. All I can do is maintain loyalty to my readers and hope they do the same to me. And to do that, I work on my craft and put out the best book(s) possible for readers. What's all I can control.