Radical  —adj. 1 fundamental (a radical error). 2 far-reaching; thorough (radical change). 3 advocating thorough reform; holding extreme political views; revolutionary. 4 forming the basis; primary. 5 of the root of a number or quantity. 6 (of surgery etc.) Seeking to ensure the removal of all diseased tissue. 7 of the roots of words. 8 bot. Of the root. —n. 1 person holding radical views or belonging to a radical party. 2 chem. A = *free radical. B atom or a group of these normally forming part of a compound and remaining unaltered during the compound's ordinary chemical changes. 3 root of a word. 4 math. Quantity forming or expressed as the root of another.  radicalism n. Radically adv. [latin: related to *radix]