The individual defines success on his or her own terms. For some, it's financial and material wealth. For others, it's about being the best version of themselves they can be. For some, it's about being really good at something and getting paid to put their passion into practice. For most, it's about leaving a legacy of sorts behind and impacting others.
The list goes on.
Whatever one's understanding of success is, there is a universal truth buried within all of these definitions:
Success is a process and a journey, not a destination. We don't reach a place or a level of success. We see and reap the fruits of labor, but we succeeded to get there. Replace "succeeded" with "walked" or "ran" or "struggled" etc. and you'll get where I'm coming from.
Are we more concerned with the fruit or the journey?