v. t. [ imp. & p. p. Countersunk p. pr. & vb. n. Countersinking. ] 1. 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. [ 1913 Webster ]

2. To cause to sink even with or below the surface; as, to countersink a screw or bolt into woodwork. [ 1913 Webster ]

n. 1. An enlargement of the upper part of a hole, forming a cavity or depression for receiving the head of a screw or bolt. [ 1913 Webster ]

☞ In the United States a flaring cavity formed by chamfering the edges of a round hole is called a countersink, while a cylindrical flat-bottomed enlargement of the mouth of the hole is usually called a conterbore. [ 1913 Webster ]

2. A drill or cutting tool for countersinking holes. [ 1913 Webster ]