countersink

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. A hole with the top part enlarged so that the head of a screw or bolt will lie flush with or below the surface.
2. n. A tool for making such a hole.
3. v. To make a countersink on or in.
4. v. To drive (a screw or bolt) into a countersink.

Century Dictionary and Cyclopedia

1. To form by drilling or turning, as a cavity in timber or other materials, for the reception of the head of a bolt or screw, a plate of iron, etc., in order that it may be nearly or quite flush with the surface: as, to countersink a hole for a screw.
2. To cause to sink in any other body so as to be nearly or quite flush with its surface: as, to countersink a screw or bolt by making a depression for its head.
3. n. A drill or brace-bit for countersinking, variously made, according as it is to be used on wood, iron, brass, etc. Specifically— A boring-bit having a conical or spherical cutter, used to make a depression to receive the head of a screw.
4. n. An enlargement of a hole to receive the head of a screw or bolt.
5. n. The recess in the chamber of a gun into which the rim of the cartridge fits.

Wiktionary

1. n. a conical recess, typically machined around a hole to admit a screw so that it sits flush with a surface.
2. v. transitive to create such a conical recess.

GNU Webster's 1913

1. v. To chamfer or form a depression around the top of (a hole in wood, metal, etc.) for the reception of the head of a screw or bolt below the surface, either wholly or in part.
2. v. To cause to sink even with or below the surface.
3. n. An enlargement of the upper part of a hole, forming a cavity or depression for receiving the head of a screw or bolt.
4. n. A drill or cutting tool for countersinking holes.

WordNet 3.0

1. n. a bit for enlarging the upper part of a hole
2. v. insert (a nail or screw below the surface, as into a countersink)
3. n. a hole (usually in wood) with the top part enlarged so that a screw or bolt will fit into it and lie below the surface