Wikipedia
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.