coun·ter·sink 
(kountər-sĭngk′)
(kountər-sĭngk′)n.
1. A hole with the top part enlarged so that the head of a screw or bolt will lie flush with or below the surface.
2. A tool for making such a hole.
tr.v. coun·ter·sunk (-sŭngk′), coun·ter·sink·ing, coun·ter·sinks
1. To make a countersink on or in.
2. To drive (a screw or bolt) into a countersink.
countersinksingle-flute countersink bit