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Definition of radical
1(especially 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 2advocating or based on thorough or complete political or social reform; representing or supporting an extreme section of a political party:a radical American activist 3of or relating to the root of something, in particular. Mathematics of the root of a number or quantity. denoting or relating to the roots of a word. Music belonging to the root of a chord. Botany of, or springing direct from, the root or stem base of a plant. 4 [usually as exclamation] informal very good; excellent:Okay, then. Seven o’clock. Radical!
1a 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 Chemistry a group of atoms behaving as a unit in a number of compounds. See also free radical.
[ early 19th century: from French]
3the 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 Mathematics a quantity forming or expressed as the root of another.