counterbore

from The Century Dictionary.

• To bore out (a hole) to a larger diameter for a given depth.
• noun A tool-holder or tool-bar for a drilling-machine.
• noun A tool for boring out a portion of a hole to a larger diameter.
• noun An enlarged section of a drilled hole, made for the purpose of receiving the cylindrical head of a screw or some similar object.

from the GNU version of the Collaborative International Dictionary of English.

• noun A flat-bottomed cylindrical enlargement of the mouth of a hole, usually of slight depth, as for receiving a cylindrical screw head.
• noun 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.
• transitive verb To form a counterbore in, by boring, turning, or drilling; to enlarge, as a hole, by means of a counterbore.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A cylindrical recess, typically machined around a hole to admit a screw so that it sits flush with a surface.
• noun The tool with which a counterbore is machined.
• verb transitive To create such a cylindrical recess.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a bit for enlarging the upper part of a hole