The noun 'heresy' was originally used - and is still most naturally used - in the context of religion. The countable noun 'a heresy' is a religious belief or doctrine which conflicts with an established belief or doctrine, i.e., with what is considered to be the correct belief or doctrine. (Obviously, what is regarded as heresy by one denomination may be regarded as orthodoxy by another, and indeed sometimes new denominations arose on the foundation of what the old denomination regarded as heresy.) The word is particularly at home in the history of the Early Christian Church, when a disputed belief or doctrine was examined by an ecumenical council, i.e., a worldwide council of the bishops of the Church, and, if considered false, formally condemned as a heresy: those holding such a belief (i.e., heretics) were expelled from the Church and, once Christianity became the official religion of the Roman Empire, were sometimes liable to criminal prosecution. For more on this see Christian Heresies.
Heresy can also be used as a non-count noun. "That is heresy!" is now a statement of general disapproval and disagreement, rather than actual disbelief; formerly, it was a statement that the disputed idea was so bad that it deserved condemnation: in the time of the Reformation, even to the punishment of being burned alive.
However, the word 'heresy' may also be used figuratively outside a religious context in reference to any unorthodox or unconventional belief or opinion. For example, if a Conservative politician were to express the view that the government should levy a high rate of tax on the profits of small businesses, his view would be regarded by his fellow-Conservatives as heresy. Or if a member of a family which had always supported a particular football team were to express the opinion that another team was better, he might be told 'That's heresy in this family'. Outside the religious context the word is sometimes used, as in the last example, in a slightly jocular way.
The word 'heresy' is pronounced with the stress on the first syllable HE-ri-si, IPA: /ˈhɛr ə (or ɪ)s ɪ/. The adjective from 'heresy' is 'heretical' - pronounced with the stress on the second syllable (hi-RE-tikl, IPA: /hɛr ˈer ɛt ɪkəl/ - and a person who holds a heretical belief is a heretic - pronounced with the stress on the first syllable HE-ri-tik, IPA: /ˈhɛr ɪ (or ə)t ɪk/.
- Etymological note: these words come originally from the Greek αἳρεσις‚ (hairesis), the basic meaning of which is: choosing or choice. However, αἳρεσις came to be used to refer to a chosen course of action or thought, or a chosen system of principles, and thence to those who adopt such principles, i.e., a philosophical school, religious group, or sect. Early on, heretic came to be 'one who has made a choice in opposition to the orthodox [or accepted]'.