radical

Concise Oxford English Dictionary © 2008 Oxford University Press:
radical/ˈradɪkl/

▶adjective

• 1 relating to or affecting the fundamental nature of something.

  ■ innovative or progressive.

• 2 (of surgery) thorough and intended to be completely curative.

• 3 advocating thorough political or social reform; politically extreme.

  ■ Brit. historical belonging to an extreme section of the Liberal party in the 19th century.

• 4 Mathematics of the root of a number or quantity.

• 5 denoting or relating to the roots of a word.

• 6 Music belonging to the root of a chord.

• 7 Botany of or coming from the root or stem base of a plant.

• 8 N. Amer. informal excellent.

▶noun

• 1 an advocate of radical political or social reform.

• 2 Chemistry a group of atoms behaving as a unit in a number of compounds. See also free radical.

• 3 the root or base form of a word.

  ■ any of the basic set of Chinese characters constituting semantically or functionally significant elements in the composition of other characters.

• 4 Mathematics a quantity forming or expressed as the root of another.

– derivatives
radicalism noun,
radicalization (or radicalisation) noun,
radicalize (or radicalise) verb,
radically adverb,
radicalness noun.

– origin ME: from late L. radicalis, from L. radix, radic- ‘root’.