radical adj
1.concerning the most basic and important parts of sth; thorough and complete: the need for radical changes in education ◆ demands for radical reform of the law ◆ radical differences between the sexes
2.new, different and likely to have a great effect: radical ideas
◆ a radical solution to the problem ◆ radical proposals / measures
3. in favour of thorough and complete political or social change: the radical wing of the party
◆ radical politicians / students / writers (AmE, slang) very good
[@more@]radically adv: The new methods are radically different from the old.
◆ Attitudes have changed radically.

noun
a person with radical opinions: political / religious radicals