A bolt-coupling was a form of joint. It held hull plating onto the outer hull of an NX-class starship. About three hundred bolt-couplings were beneath each hull section.
When Enterprise NX-01 detached a hull section from the rest of the ship in 2152, the bolt-couplings were necessarily first removed by engineering officers from the ship. (ENT: "Minefield")
Shortly afterwards, while pretending to be dissatisfied with repair work carried out on Enterprise by an automated repair station (including replacement of the detached hull section), Enterprise Chief Engineer Charles Tucker III told the space station's computer he had viewed bolt-couplings the station had used to attach the replacement hull plating. According to Tucker, the couplings weren't "up to Starfleet specs." (ENT: "Dead Stop")