In John Siracusa's extraordinary, voluminous review of Mac OS Lion at Ars Technica, he pulls out the definition of skeuomorph from Wikipedia.
A skeuomorph is a derivative object that retains ornamental design cues to a structure that was necessary in the original. Skeuomorphs may be deliberately employed to make the new look comfortably old and familiar, such as copper cladding on zinc pennies or computer printed postage with circular town name and cancellation lines. An alternative definition is “an element of design or structure that serves little or no purpose in the artifact fashioned from the new material but was essential to the object made from the original material.”
The bold emphasis is his.