A bounty hunter was a person that captured criminals for material gain. Usually bounty hunters had to be trained in security operations to be effective.
Q'Naabians had no qualms using bounty hunters within ISC borders, hunting down criminals "dead or alive", but because of law, Q'Naabian bounty hunters were not allowed to give chase to criminals in Hydran or Lyran space. Bounties were usually worth more for alive criminals than dead criminals, though. (Star Trek: False Vacuum: "Pothole Omnium")