The Collaborative International Dictionary of English v.0.48:

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.
      [1913 Webster]

   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.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

WordNet (r) 3.0 (2006):

counterbore
    n 1: a bit for enlarging the upper part of a hole [syn:
         counterbore, countersink, countersink bit]