What is the term for a flat-bottomed enlargement for a hole that allows the top of a fastener to be flush with the material's surface?

The term for a flat-bottomed enlargement of a hole, which allows the top of a fastener to be flush with the surface of the material, is indeed called a counterbore. This feature is specifically designed to provide a recessed area for the head of a screw or bolt, ensuring that it does not protrude above the surface. This is particularly useful for aesthetic purposes or when a smooth surface is required.

In contrast, a countersink creates an angled recess that allows for the head of a fastener to sit flush with or below the surface as well, but it does so with a conical shape rather than a flat bottom. A chamfer is simply an angled edge that is cut to remove the sharp corner of a part, and it does not specifically accommodate fasteners in the manner that a counterbore does. Detail View refers to a drawing or illustration that provides a closer look at a part of a design, but it does not pertain to the specific feature involved with fastener installation.