Definition of counterbore:
(n) :
A cylindrical recess, typically machined at the mouth of a hole to admit a screw (such as a cap screw) so that the screw sits flush with a surface.
(v) :
(transitive) To create such a cylindrical recess.
(n) :
The cutter used to cut such a recess. (Also used, at less depth, for spotfacing.)