The Suda or Souda ({{lang-el|Σοῦδα}, also Σουΐδας, Suidas) is a massive 10th century Byzantine Greek historical encyclopedia of the ancient Mediterranean world. It is an encyclopedic lexicon 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 proper name of the author.
The Suda is somewhere between a grammatical dictionary and an encyclopedia in the modern sense. It explains the source, derivation, and meaning of words according to the philology of its period, using such earlier authorities as Harpocration and Helladios. There is nothing especially important about this aspect of the work. It is the articles on literary history that are valuable. These entries supply details and quotations from authors whose works are otherwise lost. They use older scholia to the classics (Homer, Thucydides, Sophocles, etc.), and for later writers, Polybius, Josephus, the Chronicon Paschale, George Syncellus, George Hamartolus, and so on.
This lexicon represents a convenient work of reference for persons who played a part in political, ecclesiastical, and literary history in the East down to the tenth century. The chief source for this is the encyclopedia of Constantine VII Porphyrogenitus (912-59), and for Roman history the excerpts of John of Antioch (seventh century). Krumbacher (Byzantinische Literatur, 566) counts two main sources of the work: Constantine VII for ancient history, and Hamartolus (Georgios Monachos) for the Byzantine age.
It includes numerous quotations from ancient writers; the scholiasts on Aristophanes, Homer, Sophocles and Thucydides are also much used. The biographical notices, the author tells us, are condensed from the Onomatologion or Pinax of Hesychius of Miletus; other sources were the excerpts of Constantine Porphyrogenitus, the chronicle of Georgius Monachus, the biographies of Diogenes Laërtius and the works of Athenaeus and Philostratus.
The work deals with biblical as well as pagan subjects, from which it is inferred that the writer was a Christian. A prefatory note gives a list of dictionaries from which the lexical portion was compiled, together with the names of their authors. Although the work is uncritical and probably much interpolated, and the value of the articles is very unequal, it contains much information on ancient history and life.