The point aboard a ship where the artificial gravity cancels out, which Travis Mayweather sought out on Enterprise NX-01 so that he could sit upside down on the ceiling.
The sweet spot is the name spacers give to the place on a ship where the pull of the gravity plating falls away, usually about midway between the grav generator and the bow. Anyone who finds it can float or sit on the ceiling.
It is introduced in the Enterprise pilot Broken Bow. Tucker comes across Mayweather sitting upside down in a maintenance space, and the helmsman explains that every ship has such a spot and that he learned to find them as a child on his family's freighter, the Horizon.
The short scene establishes Mayweather as a boomer, born and brought up in space and more at home there than his Earth-raised shipmates.