\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.

\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.