The Collaborative International Dictionary
a conterbore
Countersink \Coun"ter*sink`\, n.
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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.
A drill or cutting tool for countersinking holes.