rad·i·cal / ˈradikəl/ • adj. 1. (esp. of change or action) relating to or affecting the fundamental nature of something; far-reaching or thorough: a radical overhaul of the existing regulatory framework. ∎  forming an inherent or fundamental part of the nature of someone or something: the assumption of radical differences between the mental attributes of literate and nonliterate peoples. ∎  (of surgery or medical treatment) thorough and intended to be completely curative. ∎  characterized by departure from tradition; innovative or progressive: a radical approach to electoral reform.2. advocating thorough or complete political or social reform; representing or supporting an extreme section of a political party: a radical American activist. ∎  (of a measure or policy) following or based on such principles.3. of or relating to the root of something, in particular: ∎  Math. of the root of a number or quantity. ∎  denoting or relating to the roots of a word. ∎  denoting the semantic or functional class of a Chinese character. ∎  Mus. belonging to the root of a chord. ∎  Bot. of, or springing direct from, the root or stem base of a plant.4. [usu. as interj.] inf. very good; excellent: Okay, then. Seven o'clock. Radical!• n. 1. a person who advocates thorough or complete political or social reform; a member of a political party or part of a party pursuing such aims.2. Chem. 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 214 Chinese characters constituting semantically or functionally significant elements in the composition of other characters and used as a means of classifying characters in dictionaries.4. Math. a quantity forming or expressed as the root of another. ∎  a radical sign.DERIVATIVES: rad·i·cal·ism / -ˌlizəm/ n. (in sense 1 of the noun ).rad·i·cal·ly / -ik(ə)lē/ adv. a radically different approach.rad·i·cal·ness n.

radical advocating thorough or complete political or social reform; representing or supporting an extreme or progressive section of a political party; the word in this sense is first recorded in Shelley' Oedipus Tyrannus (1820), ‘Kings and laurelled Emperors, Radical butchers’. In a letter of 1832, John Stuart Mill refers to, ‘Several friends of mine, radical-utilitarians of a better than the ordinary sort.’

Radical is recorded from late Middle English (in the senses ‘forming the root’ and ‘inherent’), and comes ultimately from Latin radix, radic- ‘root’.
radical chic a fashionable affectation of radical left-wing views or an associated style of dress or life; term coined by the American writer Tom Wolfe (1931– ).

radical pert. to the moisture inherent in animals and plants XIV; (math., philol., etc.) pert. to a root or radix; inherent, fundamental XVI; going to the root or origin, thorough XVII (r. reform XVIII); sb. radical element XVII; advocate of ‘radical reform’ XIX. — late L. rādīcālis, f. L. rādīx, rādīc- root; see -AL1.
Hence radicalism XIX.

radical Applied to a leaf that arises from a rhizome or from the base of a stem.