Hesychius of Alexandria (Ancient Greek: Ἡσύχιος ὁ Ἀλεξανδρεύς, romanized: Hēsýchios ho Alexandreús, lit. 'Hesychios the Alexandrian') was a Greek grammarian who, probably in the 5th or 6th century AD, compiled the richest lexicon of unusual and obscure Greek words that has survived, probably by absorbing the works of earlier lexicographers.
The work, titled "Alphabetical Collection of All Words" (Συναγωγὴ Πασῶν Λέξεων κατὰ Στοιχεῖον, Synagōgē Pasōn Lexeōn kata Stoicheion), includes more than 50,000 entries, a copious list of peculiar words, forms and phrases, with an explanation of their meaning, and often with a reference to the author who used them or to the district of Greece where they were current. Hence, the book is of great value to the student of the Ancient Greek dialects and in the restoration of the text of the classical authors generally – particularly of such writers as Aeschylus and Theocritus, who used many unusual words. Hesychius is important, not only for Greek philology, but also for studying lost languages and obscure dialects of the Balkans in antiquity (such as Albanoid and Thracian) and in reconstructing Proto-Indo-European. Many of the words that are included in this work are not found in surviving ancient Greek texts.