Docking latch.
A docking latch is a structural device found on Galaxy class starships. The latches serve to create the bond between the saucer module and the stardrive section and keep the two separable components joined during standard flight. There are eighteen such latches, all located on the dorsal head of the stardrive section. Their matching ports are found in the docking cavity on the saucer module.
During a saucer separation, the latches retract from their apertures and become nearly flush with the stardrive section's head, severing the link between the two components of the ship. During a reconnection, the latches work in reverse. (TNG: "Encounter at Farpoint," "The Arsenal of Freedom," "The Best of Both Worlds, Part II," "Disaster")
The docking latches are sometimes less specifically referred to as docking clamps. (TNG: "Hollow Pursuits")