The Collaborative International Dictionary of English v.0.48:

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.
      [1913 Webster]

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

The Collaborative International Dictionary of English v.0.48:

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.
      [1913 Webster]

   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.
         [1913 Webster]

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

WordNet (r) 3.0 (2006):

countersink
    n 1: 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
    2: a bit for enlarging the upper part of a hole [syn:
       counterbore, countersink, countersink bit]
    v 1: insert (a nail or screw below the surface, as into a
         countersink) [syn: set, countersink]