In binding and finishing, any narrow rule, line, or other ornamentation, especially when added to the spine of a book. The term fillet also refers to any tool (such as a hand-held wheel) used to produce such a design.
In graphics, a fillet is any rounded corner.
In typography, the term fillet refers to any arc used to connect two perpendicular lines of a border, providing a rounded edge, or a round corner connecting serifs to main strokes on letters. In this case, it is occasionally spelled filet.