Eh, I think we'd be getting into No True Scotsman territory if we tried to define TERFs as being anything other than a subgroup of radical feminism. Their whole deal is that they use feminist arguments ("our concept of 'womanhood' is molded into us at a young age by a patriarchal system that prevents women from achieving their full potential, and anyone who didn't grow up in that environment can't truly understand it") to arrive at an anti-trans position ("therefore trans women aren't really women because they weren't oppressed the way we were"). It's obnoxious, but I think we shouldn't shy away from taking ownership of our extremists. (By "we," I mean left-ish/pro-women/pro-trans/etc. people in general.)