Final answer:
St. Thomas Aquinas defined heresy as a deviation from the orthodox teachings of the Christian Church, where a heretic is someone who subscribes to beliefs outside the official doctrine.
Step-by-step explanation:
St. Thomas Aquinas's definition of heresy refers to a belief or opinion that deviates from the established orthodox teachings of the Christian Church. Aquinas, working within an Aristotelian framework adapted to Christian theology, sought to explain and defend the core elements of Christian orthodoxy against those that presented contrary views. Heretics, according to the understanding of Aquinas and the Church, were individuals who chose to uphold beliefs that went against official church doctrine, which had been defined over time through ecumenical councils and theological consensus.
The formation of this doctrine itself began early in Christian history, as theological debates around the nature of Christ and the Holy Trinity arose, creating the need to define 'correct belief' or orthodoxy. These definitions inherently produced the category of heresy by delineating which interpretations of Christianity were considered acceptable and which were not. Hence, a heretic in Aquinas's view was someone who goes against these official teachings, subscribing to or espousing theological choices outside the sanctioned scope of Christian doctrine.