異端: belief or opinion contrary to orthodox religious doctrine – Oxford English dictionary.

From the 定義, you can see 異端 is depended on orthodoxy. To define 異端, you need to define orthodoxy. orthodoxy is the reference point for 異端.  So

• To Catholicism, Protestantism is 異端.
• To Protestantism, Catholicism is 異端.
• To Lutheranism, Calvinism is 異端.
• To Baptist’s Sacramental Theology,  Lutheran’s Sacramental Theology is 異端.

So on, so forth.
Calling someone 異端 without defining orthodoxy is no different than calling someone illegal without referring which law he/she is breaking.

There’s a saying:

“Called as異端 by nondenominational Christian is no difference than called as “traitor”by internationalist  (someone not able (or unwilling) to define a border).

So instead of calling each other 異端 all the time, why don’t you guys get together to define what’s orthodoxy?
If Protestantism history of the past 400 years is of any guide…, you will know the answer to my suggestion:))

nngzh, Are you still interested the Catholic Definition of Heresy?  I can certainly give the Definition.  But why do you care what the catholic church teaches?