Counterbore

1. v. t. To form a counterbore in, by boring, turning, or drilling; to enlarge, as a hole, by means of a counterbore.
2. n. A flat-bottomed cylindrical enlargement of the mouth of a hole, usually of slight depth, as for receiving a cylindrical screw head.
3. n. 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.

The word meanings were obtained from OPTED(The Online Plain Text English Dictionary), which is based on “The Project Gutenberg Etext of Webster’s Unabridged Dictionary” which is in turn based on the 1913 US Webster’s Unabridged Dictionary (See Project Gutenburg), as a text file.