A genizah is a store-room in a synagogue where old worn-out or disused manuscripts are consigned if there is any chance that they might contain the holy name of God, YHWH, which would be exposed to desecration if the manuscript were simply thrown out. In most places the contents are taken out from time to time and reverently buried.
However, at the old synagogue in Cairo, which dates back to soon after the city’s foundation in the 9th century CE, the contents of the genizah were simply allowed to accumulate over centuries. It came to the attention of visitors from Europe in the 18th and 19th centuries, and most of the contents were cleared out and sold to Rabbi Solomon Schechter from Cambridge, England in 1896. It was treasure trove. There were several hundred thousand fragments, mostly of medieval date. Many contained parts of the Hebrew Bible. Others contained previously unknown literary works such as the Hebrew text of Sirach (Ecclesiaticus), previously only known in Greek and Latin translations, and the ‘Damascus Document’, which later turned out to be related to the Dead Sea Scrolls. Others were legal documents, letters and so forth, which between them give a superb view of ordinary life in the Jewish communities of the Mediterranean area in the 10th to the 13th centuries.
The majority of the fragments are now kept in the University Library at Cambridge. Many others are in other libraries around the world, including the Bodleian in Oxford and the John Rylands Library in Manchester.
Hoffman, Adina, and Peter Cole. Sacred Trash: The Lost and Found World of the Cairo Geniza. New York: Knopf Doubleday Publishing Group, 2016.
Rabbi Glickman, Mark. Sacred Treasure — The Cairo Genizah: The Amazing Discoveries of Forgotten Jewish History in an Egyptian Synagogue Attic. Woodstock, VT: Jewish Lights Publishing, 2011.