1. countersink - Noun
2. countersink - Verb
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; as, to countersink a hole for a screw.
To cause to sink even with or below the surface; as, to countersink a screw or bolt into woodwork.
An enlargement of the upper part of a hole, forming a cavity or depression for receiving the head of a screw or bolt.
A drill or cutting tool for countersinking holes.
Source: Webster's dictionaryIf any old tiles are loose, nail them in place with 6D nails, then use a nail set to countersink the nail heads. Source: Internet
There are also combined spotting-countersink drills which can create a screw clearance hole and countersink in one operation. Source: Internet