Heresy is anything that is contrary to what is an orthodox belief by the church. A belief being heresy does not determines its truthfulness. Martin Luther held to heresy when he went against the Roman Catholic church. Luther was right. Never the less what he held to was heresy. Further, the word heresy can be rightly used in other settings not religious in nature such as politics. This is according to the dictionary definition.
heresy
noun
her·e·sy | \ ˈher-ə-sē , ˈhe-rə-\
plural heresies
Definition of heresy
1: adherence to a religious opinion contrary to church dogma
: denial of a
revealed truth by a baptized member of the Roman Catholic Church
: an opinion or doctrine contrary to church dogma
Definition of HERESY
Scripture doesn't use even a form of the word heresy. What it does say is that gospels or Jesus that do not match up with what scripture says are called "another gospel" or "another Jesus". Based on all of this it is my view that the word heresy gets misused and has become a pejorative for views that differ from the one someone has who uses it, rather than being used according to its true definition.
People will likely continue to misuse it because it is their favorite go to pejorative but reasonable people will understand what they are doing.