adjective
1. of or relating to roots, origins, or fundamental characteristics; basic.
There are radical differences between these two systems of government.
2. extreme or complete, as a particular action or behavior.
The parents’ divorce brought about a radical change in the lives of the children.
His ideas were considered radical, and few people took him seriously.
noun
a person who advocates fundamental or extreme social and political changes.
Some of the radicals were arrested and put it jail.