Source : Webster's Revised Unabridged Dictionary (1913)

Counterbore \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.

Counterbore \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.

Source : WordNet®

counterbore
     n : a bit for enlarging the upper part of a hole [syn: {countersink},
          {countersink bit}]