Per the Area51 FAQ, the phase after Definition is:
Commit! When a proposal enters the "Commitment" phase, we will present a petition for the site's creation. Interested users are asked to digitally "sign" the proposal with their full name to help assure that site will have an active community in those critical early days. While your full name is never shown, you may add an optional comment which is displayed alongside your username in the list of committers. To ensure that commitment is taken seriously, you may only commit to three sites at any one time.
Once you've fulfilled your commitment on a new site by asking and answering a few questions, you'll be able to commit to another site. If you are very active, you may be able to re-use your commitment as soon as the second day of public beta. If you aren't active on the site, you won't be able to re-use the commitment until the end of the public beta, or until six months after the beta began.
(And the phase after that, in case you're wondering, is Beta). However, is it really necessary to explain what the next phase is right there? If a proposal is still sitting in Definition, it still needs to get the followers and questions it needs to move into Commitment, so it doesn't really matter at that point what's next, until it gets past the current thing blocking it.