a flat-bottomed cylindrical enlargement of the mouth of a hole, usually of slight depth, as for receiving a cylindrical screw head.-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.-to form a counterbore in, by boring, turning, or drilling; to enlarge, as a hole, by means of a counterbore.