A working definition of heresy

My working definition of heresy is:

Heresy is a judgement, arrived at after due process, that certain specified teaching which purports to be in accord with the teaching and standards of a particular church is in fact incompatible and unacceptable.

Heresy in the Roman Catholic Church has a structure and trajectory wholly distinct from that of protestant churches. 

A common narrative of heresy begins with the observation that, in the earliest church, haireseis simply meant choice. This changed from Ireneaus and Tertullian onwards, when heresy came came to be understood as the assertion of teaching which was opposed to, or which undermined, orthodoxy. On this basis heresy is defined as "(the act of having) an opinion or belief that is the opposite of or against what is the official or popular opinion, or an action that shows that you have no respect for the official opinion." Cambridge Dictionary

However this definition narrative feeds a myth of Christianity and therefore Christian faith and theology as unified. 

There are hundreds of Christian denominations across the world, probably many thousands depending how you count (wiki). Each is responsible for regulating its own internal integrity and external relationships. Each articulates its doctrine in terms which demarcate that particular church from the rest and also, simultaneously, integrate it with the wider church across geography and history. The idea of 'one Lord, one faith, one church' is stretched extremely thin. 

Heresy is parasitic on established church structures. It therefore fissiparates along the same trajectories as orthodoxies as churches divide and proliferate. 

(Not) defining heresies

I have not presumed to decide what is and what is not heresy. I take a more pedestrian approach, looking at incidents in which (a) heresy has formally been alleged, and (b) formal action has been taken to adjudicate the allegation. This is an historical approach as opposed to a theological  enquiry.

Faith is embodied and truth created in the dynamic conjunction of belief and community, word and practice, truth and organised structures. Faith is not timeless nor uniform and neither is heresy.