1.NounA conical recess, typically machined at the mouth of a hole to admit a screw (with a matching taper) so that the screw sits flush with a surface.
2.NounThe cutter used to cut such a recess. (Also used, at less depth, for edge-breaking/deburring.)
3.VerbTo create such a conical recess.
"to countersink a hole for a screw"
4.VerbTo cause to sink even with or below the surface.