The definition phase is a fancier, and maybe ambiguous terms for what is commonly labeled as the design phase. However, I don't want to label it as such, since I think it the term design limits the application in many minds.
So...at the beginning of the definition phase, you have an idea of what you would like to accomplish, but the idea has so many paths at this point, that you haven't determined the scope required to execute it.
There are two ways you can work through the definition phase, and I liken this to the software development design processes (waterfall vs. iterative).