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.