1. of or going to the root or origin; fundamental: a radical difference.
2. thoroughgoing or extreme, especially as regards to change from accepted or traditional forms: a radical change in the policy of the government.
3. favoring drastic political, economic, or social reforms: radical ideas; radical reform.
4. existing inherently in a thing or person: radical strengths of character.
5. slang: excellent, cool.
noun: A person who advocates thorough or complete political or social reform.
“The word ‘radical’ derives from the Latin word for root. Therefore, if you want to get to the root of anything you must be radical.” — Gore Vidal