counterbore

Short definition from Wiktionary

Noun

1. 1
   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.
2. 2
   The cutter used to cut such a recess. (Also used, at less depth, for spotfacing.)

Verb

1. 1
   To create such a cylindrical recess.