The sustained, court- and elite-sponsored effort, centered in Abbasid Baghdad from roughly the eighth to the tenth century, to translate Greek scientific and philosophical works (along with some Persian and Indian material) into Arabic — drawing especially on Syriac-speaking Christian translators, alongside Muslim, Jewish, and other scholars, enabled by cheap paper and Abbasid wealth, and feeding the scholarship later called the Islamic Golden Age, much of which returned to Latin Europe centuries afterward.
A wealthy man in ninth-century Baghdad could pay more for a fresh translation of a Greek medical text than a craftsman earned in a year. The buyers were caliphs, courtiers, doctors, and bankers, and the books they wanted were Greek: medicine, astronomy, geometry, logic. For roughly two centuries, the most ambitious knowledge project in the world was the work of turning the libraries of dead Greeks into living Arabic. Most of the translators were not Muslims. The effort preserved learning that might otherwise have vanished, and it built something new on top of it.