A radical is a person subscribing to a political ideology supporting massive, unmeasured, and rapid change. Radicalism is contrasted with conservatism
that advocate minimum change, reactionary
action advocating counter-change, as well as progressivism
advocating moderate/measured change.
A radical would want to chang, for example, a feudal system into a completely capitalist system, or a completely capitalist system into a communist one. Another exaple of radicalism would be a person who wanted to change an Islamic Theocracy into an Atheist state (see en.wikipedia.org/wiki/state_atheism)!