Parchment or other writing material from which the original writing has been erased.
If the original writing has been erased to make way for a second, superimposed writing, then the palimpsest is , properly speaking, a codex rescriptus, literally, "a re-written book."
In a palimpsest of the second type the original writing is called the primary script, while that over the primary script is known as the upper script.
The word comes from the Greek palimpsestos, meaning "scraped" or "rubbed again."
From: T.S. Haskett's Some Notes on Palaeography, Codicology and Diplomatics