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]