counterbore

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.

The cutter used to cut such a recess. (Also used, at less depth, for spotfacing.)

Meronyms: flute, pilot (the land at the tip, which rides the hole as a guide), shank

To create such a cylindrical recess.