What is a shallow flat-bottomed recess added to the opening of a hole called?

A shallow flat-bottomed recess added to the opening of a hole is referred to as a spotface. This machining operation is utilized to create a flat surface that helps ensure that a fastener, such as a bolt or screw, has a flat area to seat against the material. Spotfacing is particularly useful in situations where the original surface of the material may be uneven or requires a specific flat area for proper assembly or functionality.

The flat-bottomed design of a spotface contrasts with other definitions in machining, such as a counterbore, which is used to enlarge the diameter of a hole at its opening while also providing a cylindrical recess. Furthermore, a sunk hole typically refers to a hole that has been specifically deepened or depressed, while a recess could generally describe any indentation or hollowed area without necessarily indicating its flat-bottomed feature. Hence, spotface is the most accurate term for a shallow flat-bottomed recess.