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.
☞ 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.