A bar bill or bar tab was an account of debt owed to a bar by a patron for goods and services.
While under the impression that Julian Bashir and Miles O'Brien were dead, Quark gave a toast in their honor, "They were good customers. They always paid their bar bills on time." Quark could think of no greater tribute. (DS9: "Armageddon Game")
After being informed of Morn's apparent destitution at the time of his death, Quark commented that Morn had always paid his bar bills on time and that money had to have come from somewhere. (DS9: "Who Mourns for Morn?")
Benjamin Sisko, after playing a baseball game against a team of Vulcans led by Solok, bought everyone present in Quark's Bar a drink and asked Quark to put the cost on his tab. (DS9: "Take Me Out to the Holosuite")
In 2375, Worf disputed responsibility for a bar bill with Quark, but eventually paid it after Quark mentioned that it would not be good if word got out that he was refusing to honor the debts incurred by his own House. (DS9: "Prodigal Daughter")
Later that year Worf bought the crew of the IKS Koraga three barrels of bloodwine from Quark and put it on his tab. After Worf was missing following the destruction of the ship, Quark told Ezri Dax that Worf would return, as he would not want to go to Sto-vo-kor indebted to Quark. (DS9: "Penumbra")