Baseball America's 2006 Top 100 Prospect list note:
Our 17th Annual Top 100 Prospects list is based on each player's long-term major league value, a combination of his upside and his likelyhood of reaching that ceiling.
This is what you are going to get out of most prospect lists. If you were to go by ceiling, you'd get a top 10 list with guys like Mark Rogers, Clayton Kershaw, and Jacob McGee included. Is that really what you're after?
I'd go by the BA definition above unless mentioned otherwise. That's the most commonly used definition for a prospect.