The Suda or Souda (Medieval Greek: Σοῦδα Soũda) is a massive 10th-century Byzantine encyclopedia of the ancient Mediterranean world, formerly attributed to an author called Suidas. It is an encyclopedic lexicon, written in Greek, with 30,000 entries, many drawing from ancient sources that have since been lost, and often derived from medieval Christian compilers. The derivation is probably from the Byzantine Greek word souda, meaning "fortress" or "stronghold," with the alternate name, Suidas, stemming from an error made by Eustathius, who mistook the title for the author's name.