A Skeuomorph is a decorative object that retains the design of the original object when the design elements of the original are no longer needed. In physical construction, the inclusion of molded rivets, screw heads or other non-functional elements present on the original objects made of metal or wood but now made of plastic are examples of skeuomorphs.
Skeuomorphs are even more apparent in digital interfaces, however, as there is usually no reason to clone the interface of an analog device for a digital interface that goes beyond tradition or design aesthetics. One of the most obvious examples of this is the plethora of music applications that contain a digital representation of the analog interface residing on the original mixers they mimic. For example, the interface above comes from the GEARcompressor audio plugin from Redstair. [1