Counterbore

Pronunciation: -b?r`
n.

1. A flat-bottomed cylindrical enlargement of the mouth of a hole, usually of slight depth, as for receiving a cylindrical screw head.
2. A kind of pin drill with the cutting edge or edges normal to the axis; -- used for enlarging a hole, or for forming a flat-bottomed recess at its mouth.

Pronunciation: koun`t?r-b?r"
v. t.

1. To form a counterbore in, by boring, turning, or drilling; to enlarge, as a hole, by means of a counterbore.