for the reception of the head of a screw or bolt below the surface, either wholly or in part; as, to countersink a hole for a screw
The larger diameter at the entrance of a machined hole with a conical bottom Similar to a counterbore except for the bottom configuration The general use of a countersink if for use with flat head screws and bolts
a hole (usually in wood) with the top part enlarged so that a screw or bolt will fit into it and lie below the surface