Updated: Feb 1
In the common understanding it defines someone who plays successfully, in a given moment, performing in a way somehow natural or that comes with ease.
As far as experience and knowledge I would suggest to have a different approach. Let's limit the definition to "someone who performs far above the average at professional level", this is essentially what players, parents and coaches are looking for.
“There are exceptionally skilled kids who becomes top pros as well as there are exceptional kids who never make it!"
So the point is how to grow young players who show a predisposition for tennis?
There are essentially two scenarios of players who made it:
1. Players that since a young age show some extraordinary qualities and in their environment find the way to progress to become top professionals. This cases are very rare and are not the expression of a "system", this can happen wherever in the world.
Of course determination and an appropriate amount of quality training is needed to make it happen.
2. Players that are coming from "hotbeds", generally local contests in which, for different reasons, there are kind of "contagious" conditions that help/guide the explosion of more top athletes in a single spot/area.
For the first case, it will definitely be interesting to explore cases like Federer, Nadal, Djokovic, the Williams sisters, or going a big backwards Sampras, Agassi, Clijsters, Hingis, etc. and find some common elements.
It could also be interesting to study cases of kids successful at young ages, who didn't meet the expectations.