Countersink / Counterbore holes

 Countersink holes V.S. Counterbore holes

A countersink is a conical hole cut into a manufactured object, or the cutter used to cut such a hole. A common use is to allow the head of a countersunk bolt or screw, when placed in the hole, to sit flush with or below the surface of the surrounding material. By comparison, a counterbore makes a flat-bottomed hole that might be used with a socket-head capscrew.

Countersink holes vs Counterbore holes

V figure Count-sink vs T figure Count-Sink:

A countersink is a conical hole that is cut in order to allow for a flat head screw to be used.

A counterbore is a cylindrical flat-bottom hole that is cut that also allows for a socket cap screw (has a flat head) to be used.