Heresy is a term used to describe people who had opinions that differed from the doctrine of the established Church. In the late medieval and Early Modern periods, such people were called heretics. From the 1300s to the 1600s, so-called heretics were often persecuted and burnt at the stake, particularly during the witch trials in Europe. However, many people who were tortured and executed during this period were not heretics, but merely victims of a mass hysteria.

Today, the word 'heresy' is often used in a wider sense to describe an opinion that differs from any established belief or custom, be it religious or non-religious.