Absolutely flat – when one surface lies as close to and smoothly as possible against another, so that meeting points appear to be and are to the hand, seamless. Such as we see in the best marquetry and inlay, work that involves a high degree of accuracy in measuring and cutting. For our purposes its always joining fabric as one seam or another, insets, gores, patchwork, made stripes etc..
In architecture, flush doors, or jib doors, have no architectural detail. These are the sort that you need for a visually uninterrupted wall, for dressing rooms, wardrobes and corridors; they are often covered in wallpaper or fabric to match the surroundings, sometimes edged or studded,but mostly not. .