Esteban Panzera wrote an interesting blog post called 11 Tips for Appearing at Google Definitions which was shared on the SEOmoz blog. Some of her tips include creating a unique definition, using separators (such as a - or :) to separate the term from the definition, and to provide more than 5 definitions per page.
We've acknowledged in the past that definitions help search rankings in Google. If you rank #1 for the definition, you'll get a good amount of traffic -- that is, if people click on the link. A few years after that implementation, Google changed the layout of definitions so that you'd actually have to click through to see more definitions. Even so, however, definitions are good. It helps you rank for keywords you may not necessarily be targeting for that traffic boost regardless, and that can't be a bad thing.
Here's a look at two ways to get your definitions:
You can do a Google search for "what is..." and find the definition followed by your regular 10 links.
Or you can use the define: operator (my preferred method) to get a lot more definitions:
Forum discussion continues at Sphinn.