Success is hard to define. I would say it is accomplishment. Different people accomplish different things. You might want to finish a certain project, pass a certain course, get a certain car or finish a certain degree. When you set out to achieve something and achieve it, that is success.
This is a rather deep question, as success is a concept that I think is defined entirely by an individual's goals and aspirations. This means that success is going to look different depending on each person. One person's idea of success could be another person's absolute nightmare. Let me give you a little illustration to make my point. I recently got in touch with an old school friend on facebook, who is leading a very "successful" life working as a Merchant Banker in London. He has earned lots of money and has a big house in London, the second most expensive capital city in the world. Many would argue that he is a success. However, he hates his job, blames it for his divorce and on average works a 70 hour week. His life would be my idea of a nightmare, not a success. On the contrary, my much more moderate "success" is based on earning a lot less but having a job that I am happy with (most of the time) and a wife and children and having time to pursue my own interests outside of work. Do you see the difficulty in defining a concept as nebulous as success?
Hope it works! :)