Zandik is a Zoroastrian term of uncertain origin and meaning, but conventionally interpreted as 'heretic' in a narrow sense, or, in a wider sense, for a person with any belief or practice that ran contrary to Sassanid-mediated Zoroastrian orthodoxy.
The Middle Persian term engendered the better-attested Arabic zindiq, with the same semantic field but related to Islam rather than Zoroastrianism. In the Islamic world, including Islamic-era Iran, the term was also variously assigned to Manichaeans, Mandaeans, Mazdakites, Zoroastrians, Buddhists, Christians, and free-thinkers in general, including Muslims. Whether zandik was also used in any of these ways in Zoroastrian times is unknown; in that context, the term is only attested in three texts (two from the same author), and in all three appears as a hapax used in a pejorative way, but with no additional hints from which to infer a meaning.
In several now-obsolete studies related to Zoroastrianism, the word was also speculated to be the proper name of a particular (but hypothetical) priestly tradition that embraced Zurvanite doctrine.
The conventional translation as 'heretic' was already common in the 19th century when Christian Bartholomae (1885), derived zandik from Avestan zanda, which he treated as a name of certain heretics.
Early examples of Arabic zindiq primarily denote Manichaeans, and this is possibly also the meaning of the term in the early attested use in Middle Persian (see below). This early usage led A. A. Bevan to derive Middle Persian zandik from Syriac zaddiq 'righteous' as a Manichaean technical term for 'listeners' (i.e. lay persons, as contradistinguished from the Manichaean elite). Bevan's derivation was widely accepted until the 1930s, especially amongst scholars of Semitic languages, but was discredited following a comprehensive review of both Arabic and Iranian usage by H. H. Schaeder (1930). Schaeder pointed out that the substantive was zand, not zandik (an etymology would thus have to explain zand, not zandik), as -ik was merely a regular Middle Iranian adjectivizing suffix.