1. (n.) A flat-bottomed cylindrical enlargement of the mouth of a hole, usually of slight depth, as for receiving a cylindrical screw head.

2. (n.) 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.

3. (v. t.) To form a counterbore in, by boring, turning, or drilling; to enlarge, as a hole, by means of a counterbore.