What do you call the bulge in the middle of a column?

The bulge in the middle of a column is called a "Flute" or "fluting," especially in classical architecture. This feature is often decorative and refers to the shallow vertical grooves that run along the length of the column. In some contexts, the bulge may also be referred to as a "scroll" or "entasis," which is a slight curvature designed to correct optical illusions and create a more aesthetically pleasing appearance.