- opinion or doctrine at variance with the orthodox or accepted doctrine, especially of a church or religious system.
- the maintaining of such an opinion or doctrine.
- adherence to a religious opinion contrary to church dogma
- dissent or deviation from a dominant theory, opinion, or practice
- an opinion, doctrine, or practice contrary to the truth or to generally accepted beliefs or standards
Omit the references to "churches or religious systems" and you have a good understanding of what it means to be a heretic today.
The best among us are heretics about the right things.
The best heretics stand contrary to orthodoxy because they have discipline in the way they look at reality and a demanding rigor in what they will accept as a personal conviction.
Arriving into heresy can be profound. Take for instance this quote from Men in Black:
"1500 years ago everybody knew the earth was the center of the universe... 500 years ago everybody knew the world was flat... 15 minutes ago you knew that people were alone on this planet."
To be willing to call beliefs into question is a form of strength. But strength without discipline will create as much harm as benefit like a fire in a dry overgrown forest.
In the realm of beliefs, we clear out the undergrowth... or else. We call this "being a clear thinker". Basically it means you have culled away the ideas that don't conform with reality in a non-contradictory way.
We often don't think too hard about the way our minds work. We don't name things. But this process has a name. The name for the process of integrating perception of a set of facts in reality (rationality) into a non-contradictory understanding (logic) is "Reason", which is much maligned in our time. But reason, the disciplined application of rationality and logic, is the difference between folly and informed heresy.
For some of us, the accidental and proud heretics, there will never be such a thing as a "post-truth" world. We know what we believe and we know why we believe it: because our beliefs have not been overthrown by new facts and we have worked hard to avoid conveniently "not seeing" things.