Counterbore

A counter bore provides a square bottomed, recessed hole. They are usually found when a Screw#Types_of_screws is required to sit flush with the workpiece's surface.
The uppermost counterbores, as shown in the image, are of the same tool. The smaller top item is an insert, the middle image shows another three fluted counterbore insert, assembled in the holder. The shank of this holder is a machine tapers#morse although there are other machine tapers that are used in the industry.

The lower image is of a plain counter bore, this is designed to fit into Chuck (engineering)#drill and being smaller, it is economical to make it as one piece.