According to Ryu Murakami, my all-time favorite writer, success is defined by the following three criteria.

1. Have a fulfilling job.
2. It also pays enough to support your lifestyle.
3. Have a small community that you trust and fall back on.

Good, I am gonna use this definition. They sound true to me (though there are a lot of truths), and look achievable.

To know a bit about Murakami:

Steve Ericsson meets Ryu Murakami