Telegraph: Reports " A candidate is officially nominated when they gain a majority of delegates voting at the convention. When no candidate enters the convention with a simple majority - over 50 per cent - of delegates committed to them, a contested convention ensues..."
Lets Get Real:
The delegates are bound in the first ballot to the candidate that was chosen by their State and after that its open season on getting the delegates to chose the nominee. That why Trump being rich comes in to play, if the Campaign is any good it will have files on each delegate and what they would expect should they become unbound after the 1st ballot. That type of political operation takes years of being in the field and a LOT of money. Overall it might be Trump or someone we have not heard about, that is why a contested convention is fun.