Etymologicum Magnum (standard abbreviation EM) is the traditional title of a Greek lexical encyclopedia compiled at Constantinople by an unknown lexicographer around AD 1150. It is the largest Byzantine lexicon and draws on many earlier grammatical, lexical and rhetorical works. Its main sources were two previous etymologica, the so-called Etymologicum Genuinum and the Etymologicum Gudianum. Other sources include Stephanus of Byzantium, the Epitome of Diogenianus, the so-called Lexicon Αἱμωδεῖν, Eulogius’ Ἀπορίαι καὶ λύσεις, George Choeroboscus’ Epimerismi ad Psalmos, the Etymologicon of Orion of Thebes, and collections of scholia. The compilator of the Etymologicum Magnum was not a mere copyist; rather he amalgamated, reorganised, augmented and freely modified his source material to create a new and individual work.