In the context of the Middle Ages, a heretic would be most accurately defined as:
a Christian who did not follow accepted Church teachings.
Heretics were typically Christians whose beliefs or practices deviated from the doctrines established by the Church. The Church viewed heresy as a significant threat to its authority and unity, leading to various measures, including persecution, to maintain orthodoxy. While other groups mentioned (Jews, Muslims) faced discrimination and persecution, they were not categorized as heretics in the same way since heresy specifically pertains to deviations from Christian doctrine.