It can't be clearly defined, but you do know it when you see it.
Is this not a 'God-of-the-gaps' argument? Poor logic.
In fact, isn't the lack of clear definition for such an important concept...the whole darn point?
It's an UN-falsifiable assertion. There's never enough evidence to rule it out. So when an accusation is leveled...the accused is automatically guilty.
https://en.wikipedia.org/wiki/Salem_witch_trials