Countersink \Coun"ter*sink`\ (koun"t[~e]r*s[i^][ng]k`; 277), v. t. [imp. & p. p. Countersunk (-s[u^][ng]k`); 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.

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

Source: Webster's Revised Unabridged Dictionary (1913)
 

Countersink \Coun"ter*sink`\, 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.

Note: 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.

2. A drill or cutting tool for countersinking holes.

Source: Webster's Revised Unabridged Dictionary (1913)