radical (ME. < LL. radicalis < L. radix, a ROOT). -adj. (1a) of or from the roots; going to the foundation or source of something [as in a radical principle]. (1b) extreme; thorough [as in a radical change]. (2) favoring fundamental or extreme change, especially in socio-economic structure. (3) Botany: of or coming from the root. (4) Math: having to do with the root or roots of a number or quantity. -noun (1) a basic or root part of something. (2) one favoring basic change in socio-economic structure. (3) Music: same as root, the basic tone of a chord, on which the chord is constucted; often, the fundamental. (4) Chemistry: a group of two or more atoms that acts as a single atom and goes through a reaction unchanged, or is replaced by a single atom. (5) Math: the indicated root of a quantity or quantities. [ref: Webster's New World Dictionary of the American Language, Second College Edition]