of or going to the root or origin; fundamental

a radical difference.

thoroughgoing or extreme, esp. as regards change from accepted or traditional forms

a radical change in the policy of a company.

favoring drastic political, economic, or social reforms

radical ideas; radical and anarchistic ideologues.

forming a basis or foundation.

existing inherently in a thing or person

radical defects of character.

Mathematics .

pertaining to or forming a root.

denoting or pertaining to the radical sign.

irrational ( def. 5b ) .

Grammar . of or pertaining to a root.

Botany . of or arising from the root or the base of the stem.

a person who holds or follows strong convictions or extreme principles; extremist.

a person who advocates fundamental political, economic, and social reforms by direct and often uncompromising methods.

Mathematics .

a quantity expressed as a root of another quantity.

the set of elements of a ring, some power of which is contained in a given ideal.

radical sign.

Chemistry .

group ( def. 3 ) .

free radical.

Grammar . root ( def. 11 ) .

(in Chinese writing) one of 214 ideographic elements used in combination with phonetics to form thousands of different characters.

adjective

extrinsic , nonessential , superficial , conservative , moderate , derivative , slight , trifling

noun

conservative , moderate

adjective

basal , bottom , cardinal , constitutional , deep-seated , essential , foundational , inherent , innate , intrinsic , meat-and-potatoes , native , natural , organic , original , primal , primary , primitive , profound , thoroughgoing , underlying , vital , advanced , anarchistic , complete , entire , excessive , extremist , fanatical , far-out * , freethinking , iconoclastic , immoderate , insubordinate , insurgent , insurrectionary , intransigent , lawless , leftist , militant , mutinous , nihilistic , progressive , rabid , rebellious , recalcitrant , recusant , refractory , restive , revolutionary , riotous , seditious , severe , sweeping , thorough , ultra , ultraist , uncompromising , violent , way out , basic , fundamental , fanatic , drastic , extreme , heretical , heterodox , thorough-going , unconventional , unorthodox

noun

agitator , anarchist , avant-garde , extremist , fanatic , firebrand , freethinker , iconoclast , insurgent , insurrectionist , leftist , left-winger , militant , mutineer , nihilist , nonconformist , objector , pacifist , progressive , rebel , reformer , renegade , revolter , revolutionary , rioter , secessionist , subversive , ultraist , revolutionist , ultra , zealot , basal , basic , cardinal , complete , drastic , essential , etymon , extreme , fundamental , jacobin , liberal , maximalist , primal , primary , radix , root , sansculotte , septembrist , young turk