Webster's 1913 Dictionary
COUNTERBORE
COUNTERBORE Coun "ter *bore ` (-br `), n.
1. A flat-bottomed cylindrical enlargement of the mouth of a hole, usually of slight depth, as for receiving a cylindrical screw head.
2. 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.
COUNTERBORE
COUNTERBORE Coun `ter *bore " (koun `tr-br "), v. t.
Defn: To form a counterbore in, by boring, turning, or drilling; to enlarge, as a hole, by means of a counterbore.
New American Oxford Dictionary
counterbore
coun ter bore |ˈkountərˌbôr ˈkaʊntərbɔr | ▶noun a drilled hole that has a flat-bottomed enlargement at its mouth. • a drill whose bit has a uniform smaller diameter near the tip, for drilling counterbores in one operation. ▶verb [ with obj. ] drill a counterbore in (an object ).
Oxford Dictionary
counterbore
coun ¦ter |bore |ˈkaʊntəbɔː | ▶noun a drilled hole that has a wider section at the top. • a drill whose bit has a uniform smaller diameter near the tip, for drilling counterbores in one operation. ▶verb [ with obj. ] drill a counterbore in (an object ).
The Wordsmith Dictionary