Heresy is a belief or opinion that goes against the official teachings of a religious group, especially when someone who belongs to that religion publicly disagrees with its core doctrines. The word comes from a Greek term meaning “choice,” because a heretic chooses to believe something different from what their religious leaders say is true.

Throughout history, religious authorities have taken heresy very seriously. In medieval Europe, the Catholic Church considered certain beliefs heretical and sometimes punished people severely for holding them. For example, saying that the Earth orbited the Sun was once considered heresy because some church officials believed the Earth stood still at the center of the universe. Scientists like Galileo faced serious trouble for teaching ideas that contradicted church doctrine.

Different religions define heresy differently. What counts as heresy in one faith might be perfectly acceptable in another. The key is that heresy happens within a religion: you can't commit heresy against beliefs you never claimed to follow in the first place.

Today, people sometimes use heresy playfully outside of religion. A baseball fan might jokingly call it “heresy” when someone suggests that a rival team's player is better than their favorite. In this casual usage, it just means an opinion that goes against what a group believes strongly.