\Coun"ter*bore`\ (-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.

\Coun`ter*bore"\ (koun`t?r-b?r"), v. t.
To form a counterbore in, by boring, turning, or drilling; to
enlarge, as a hole, by means of a counterbore.