Zahhāk or Zahāk (zæhɒːk) (ضحّاک), also known as Zahhak the Snake Shoulder (Zahhāk-e Mārdoush), is an evil figure in Persian mythology, evident in ancient Persian folklore as Azhi Dahāka (اژی دهاک), the name by which he also appears in the texts of the Avesta. In Middle Persian he is called Dahāg (دهاگ) or Bēvar Asp (بیور اسپ) the latter meaning "he who has 10,000 horses". In Zoroastrianism, Zahhak (going under the name Aži Dahāka) is considered the son of Ahriman, the foe of Ahura Mazda. In the Shāhnāmah of Ferdowsi, Zahhāk is the son of a ruler named Merdās.
Aži (nominative ažiš) is the Avestan word for "serpent" or "dragon". It is cognate to the Vedic Sanskrit word ahi, "snake", and without a sinister implication.
The original meaning of dahāka is uncertain. Among the meanings suggested are "stinging" (source uncertain), "burning" (cf. Sanskrit dahana), "man" or "manlike" (cf. Khotanese daha), "huge" or "foreign" (cf. the Dahae people and the Vedic dasas). In Persian mythology, Dahāka is treated as a proper noun, while the form Zahhāk, which appears in the Shāhnāme, was created through the influence of the unrelated Arabic word ḍaḥḥāk (ضَحَّاك) meaning "one who laughs".
The Avestan term Aži Dahāka and the Middle Persian aždahāg are the source of the Middle Persian Manichaean demon of greed Až, Old Armenian mythological figure Aždahak, Modern Persian 'aždehâ/aždahâ', Tajik Persian 'ažhdahâ', Urdu 'ažhdahā (اژدها), as well as the Kurdish ejdîha (ئەژدیها) which usually mean "dragon".
The name also migrated to Eastern Europe, assumed the form "ažhdaja" and the meaning "dragon", "dragoness" or "water snake" in Balkanic and Slavic languages.
Despite the negative aspect of Aži Dahāka in mythology, dragons have been used on some banners of war throughout the history of Iranian peoples.
The Ažhdarchid group of pterosaurs are named from a Persian word for "dragon" that ultimately comes from Aži Dahāka.
Aži Dahāka is the most significant and long-lasting of the ažis of the Avesta, the earliest religious texts of Zoroastrianism.