What is Heresy?

Legal Definition
Heresy is any belief or theory that is strongly at variance with established beliefs or customs. A heretic is a proponent of such claims or beliefs. Heresy is distinct from both apostasy, which is the explicit renunciation of one's religion, principles or cause, and blasphemy, which is an impious utterance or action concerning God or sacred things.

The term is usually used to refer to violations of important religious teachings, but is used also of views strongly opposed to any generally accepted ideas. It is used in particular in reference to Christianity, Judaism, and Islam.

In certain historical Christian, Islamic and Jewish cultures, among others, espousing ideas deemed heretical has been and in some cases still is subjected not merely to punishments such as excommunication, but even to the death penalty.
-- Wikipedia
Legal Definition
Eng. law. The adoption of any erroneous religious tenet, not warranted by the established church.

2. This is punished by the deprivation of certain civil rights, and by fine and imprisonment. 1 East, P. C. 4.

3. In other countries than England, by heresy is meant the profession, by Christians, of religious opinions contrary to the dogmas approved by the established church of the respective countries. For an account of the origin and progress of the laws against heresy, see Giannoni's Istoria di Napoli, vol. 3, pp, 250, 251, &c.

4. in the United State, happily, we have no established religion; there can, therefore, be no legal heresy. Vide Apostacy; Christianity.
-- Bouviers Law Dictionary
Legal Definition
In English law. An offence against religion, consisting not in a total denial of Christianity, but of some of its essential doctrines, publicly and obstinately avowed. 4 Bl. Comm. 44, 45. An opinion on divine subjects devised by human reason, openly taught, and obstinately maintained. 1 Hale, P. C. 384. This offence is now subject only to eccleslastical correction, and is no longer punishable by the secular law. 4 Steph. Comm. 233.
-- Black's Law Dictionary
Legal Definition
A denial of some of the essential doctrines of Christianity, publicly and obstinately avowed. See 4 Bl. Comm. 44.
-- Ballentine's Law Dictionary