(1):
(n.) An enlargement of the upper part of a hole, forming a cavity or depression for receiving the head of a screw or bolt.
(2):
(v. t.) To cause to sink even with or below the surface; as, to countersink a screw or bolt into woodwork.
(3):
(n.) A drill or cutting tool for countersinking holes.
(4):
(v. t.) To chamfer or form a depression around the top of (a hole in wood, metal, etc.) 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.