adj. 1.(used of opinions and actions) far beyond the norm 2.markedly new or introducing radical change 3.arising from or going to the root 4.of or relating to or constituting a linguistic root 5.especially of leaves radical n. 1.(chemistry) two or more atoms bound together as a single unit and forming part of a molecule 2.an atom or group of atoms with at least one unpaired electron 3.a person who has radical ideas or opinions 4.a character conveying the lexical meaning of a logogram 5.a sign placed in front of an expression to denote that a root is to be extracted 6.(linguistics) the form of a word after all affixes are removed