Witnessing was an practice in use in the Orion Syndicate, in which one member would vouch for the loyalty of another. The act of witnessing was not taken lightly, as, if the new member were found to be disloyal or in violation of the rules of the Syndicate, they would be killed along with the individual who had witnessed.
In 2374, Syndicate member Liam Bilby witnessed for Miles O'Brien, who was, in actuality, working undercover for Starfleet Intelligence. When Bilby eventually learned where O'Brien's true loyalties lay, and that Bilby was being set up to be killed by Starfleet Intelligence, Bilby decided to press on with the mission, realizing that, if he turned himself into protective custody, the Syndicate would target Bilby's family, but that, if it appeared that he had gone to his death unaware of O'Brien's treachery, the Syndicate might leave his family alone. Bilby's plane worked and, although he was killed, his wife continued to be supported by the Syndicate. (DS9: "Honor Among Thieves", "Prodigal Daughter")