Tillich uses the definition- one who questions, as in the Apostle Thomas. At this time when old religious ideas are ineffectual, recent discoveries unsettling, isn't it time to ask if what has been passed down as true, is such? The solution to the current messiness is not a fight to see who can be most intimidating, but an idea that makes everything fall into place. Everything that is is reasonable, we just need to find the correct one, this means questioning until one comes across that that cannot be questioned.
Aren't the roots of this word in latin? There's nothing about it, and nothing about latin meaning of this word! I don't have proper sources, but I hope someone does.
The word "heresy" comes from the Greek αἵρεσις, hairesis (from αἱρέομαι, haireomai, "choose"), which means either a choice of belief.