Palimpsest: a piece of writing material, usually parchment or leather, from which a text has been eraded to reuse it for another text. Sometimes, the first text remains legible.
Ancient writing materials were precious, especially parchment. When a text was no longer needed, a sheet could be scratched off ("eraded") to allow the writing of a new text. Sometimes, the original text is still recognizable on such a palimpsest, although it may require ultraviolet light. We speak of an old subtext and a more recent upper text.
A famous example is the Archimedes Palimpsest. Discovered in 1906, it disappeared during the First World War and was rediscovered in 1998. Below the upper text, a thirteenth-century prayer book, is Archimedes' treatise On Method.