Definition of brokered convention
A brokered convention occurs when no candidate to be the party’s nominee receives a majority of the delegates’ votes after the first round of voting at the convention. This can happen when no candidate receives a majority of delegates after each primary and caucus has been conducted.
After the first round of voting, delegates are no longer bound by the results of the state they represent. The party conducts subsequent delegates’ votes until a majority is reached, presumably with back-room deals taking place as candidates attempt to convince their opponents to drop out in a quid-pro-quo arrangement.