You're definitely correct about there being some ambiguity in the definition of radicalism.
I think there are two main indicators of radicalism. The first is the assumption that one is correct to the point that there is a complete unwillingness to even consider contrary possibilities. The second, which is often linked to the first, is a tendency to resort to violence against those do not feel the same as they do.
So for instance Jehova's Witnesses are certainly not mainstream and their views are quite extreme. But even the most devout and determined door knocker is not somebody I would call a radical. They're happy to, and indeed seem to take immense pleasure, in discussing and defending their religion even to those who would challenge and reject its very core - even if the questioner does so in less than tactful ways. While Jehova's may not necessarily change their view, they would certainly indulge alternative views. And violence is never even considered.
By contrast take those who adhere to radical Islam. In that case anything short of complete deference is seen as a grievous offense. Acts such as drawing their religious figure is seen as sufficient cause to literally murder individuals, in mass. That behavior is absurdly radicalized. In the US we're still far away from that level. But increasingly we're seeing individuals incapable of discussing things with one another, and violence is an increasingly common outcome. And as individuals become further and further ideologically isolated, this radicalism and mutual intolerance is only like to increase.