A tool or drill bit used to create a countersunk or enlarged hole, especially one that recesses fasteners below a surface.
From counter- (opposite direction) + borer (a tool that bores holes), + -er (agent noun). An industrial English term for a specialized machining tool.
A counterborer is essentially a drill bit with multiple diameters, and machinists can perform both the initial hole and the enlargement in one operation without changing tools.
Complete word intelligence in one call. Free tier — 50 lookups/day.