Heresy was a belief which opposed established doctrine, usually religious. A person who held such beliefs was called a heretic.
On Earth in the distant past, Megron angrily denounced as heresy the Fourth Doctor's prediction that order would arise out of the chaos. (AUDIO: Exploration Earth)
On Mars, Skaldak was noted as being the vanquisher of the Phobos Heresy. (TV: Cold War)
In 325, Arius was regarded as a heretic for believing that God had created Jesus Christ, which contradicted the belief that God and Jesus were part of the Trinity. (AUDIO: The Council of Nicaea)
In the 16th century, the Catholic Church regarded Protestants as heretics and persecuted them. (TV: The Massacre, AUDIO: The Marian Conspiracy)
The Catholic Church considered heliocentrism to be heretical. (AUDIO: The Galileo Trap)
On Tigella, Deon leader Lexa regarded the Savants as heretics for wanting to study the Dodecahedron, which she believed was a gift from Ti. (TV: Meglos)
On Sarn, Timanov regarded those who did not believe in Logar as heretics. (TV: Planet of Fire)
On Ribos, Binro's discovery that the change of seasons was caused by natural processes rather than the war between the gods was regarded as heresy and he was forced to recant. (TV: The Ribos Operation)
In the Church, it was a Level One Heresy, punishable by death, to attempt to lower the hood of a Headless Monk. (TV: A Good Man Goes to War)
In 5343, Scratch denounced as heresy the sight of River Song and the Twelfth Doctor presenting King Hydroflax's cybernetic head. (TV: The Husbands of River Song)
Calib called Leela's blasphemy against Xoanon a heresy. (TV: The Face of Evil)
On Adamantine, Yasmin Khan's claim that she and Graham O'Brien were from another world was denounced as heresy and they were arrested by the Greenwatch. (PROSE: Molten Heart)
When the Dalek Prime was restored during the Last Great Time War, an alternate Davros made a plea for the revived Dalek Empire to instead follow him. When one agreed, the Prime dubbed it a heretic and ordered it exterminated. (AUDIO: Restoration of the Daleks)