Intrade has a contract available on whether the Democratic convention will be "brokered". It's currently being assessed at about a 15% chance it will be brokered.
In the rules of this contract, they specify "brokered" as follows:
"If the Presidential nominee is not decided after the first round of delegate voting at the party convention the convention will be considered "brokered" (i.e. it takes multiple rounds of voting by party delegates to decide the nominee)."