A bit for enlarging the upper part of a hole

The Collaborative International Dictionary Word definitions in The Collaborative International Dictionary
Counterbore \Coun`ter*bore"\ (koun`t?r-b?r"), v. t. To form a counterbore in, by boring, turning, or drilling; to enlarge, as a hole, by means of a counterbore.

Wiktionary Word definitions in Wiktionary
n. 1 A cylindrical recess, typically machined around a hole to admit a screw so that it sits flush with a surface. 2 The tool with which a counterbore is machined. vb. (context transitive English) To create such a cylindrical recess.

WordNet Word definitions in WordNet
n. a bit for enlarging the upper part of a hole [syn: countersink , countersink bit ]

Wikipedia Word definitions in Wikipedia
A counterbore (symbol: ⌴ ) is a cylindrical flat-bottomed hole that enlarges another coaxial hole, or the tool used to create that feature. A counterbore hole is typically used when a fastener, such as a socket head cap screw , is required to sit flush ...