From Merriam-Webster:

Main Entry: rad•i•cal
Pronunciation: 'ra-di-k&l
Function: adjective
Etymology: Middle English, from Late Latin radicalis, from Latin radic-, radix root -- more at ROOT
Date: 14th century
1 : of, relating to, or proceeding from a root: as a (1) : of or growing from the root of a plant (2) : growing from the base of a stem, from a rootlike stem, or from a stem that does not rise above the ground b : of, relating to, or constituting a linguistic root c : of or relating to a mathematical root d : designed to remove the root of a disease or all diseased tissue
2 : of or relating to the origin : FUNDAMENTAL
3 a : marked by a considerable departure from the usual or traditional : EXTREME b : tending or disposed to make extreme changes in existing views, habits, conditions, or institutions c : of, relating to, or constituting a political group associated with views, practices, and policies of extreme change d : advocating extreme measures to retain or restore a political state of affairs