@admitsWrongIfProven
This is just my understanding - many of the female TERFs are radical feminists in the sense that they profess to support women and girls with great fervor. They oppose the "dilution" of rights/protections for girls and women by the supposed erasure of "womanhood" by including trans women as women.
I'm sure there are more scholarly definitions, but like I said, this is my understanding of the radical feminist part. (I think of them more as wRong feminists.) ;)