Radical is marked by a considerable departure from the usual or traditional, Extreme; tending or disposed to make extreme changes in existing views, habits, conditions, or institutions : of, relating to, or constituting a political group associated with views, practices, and policies of extreme change : advocating extreme measures to retain or restore a political state of affairs.[1]
A radical (in political history) was a member of the left-wing party in the French National Assembly and refers to many modern leftists (particularly those such as Obama-mentor and friend Bill Ayers, Bernardine Dohrn, Herbert Croly, Saul Alinsky and Bobby Seale). It is also commonly used by self-identified radical lesbian separatist feminists such as Andrea Dworkin and Angela Davis.
In mathematics a radical is a square root or other root.