If something is considered extremist or very different from anything that has come before it, call it radical.

The noun, radical, comes from the Latin radix "root," and in fact, radical and root are synonymous as technical terms in fields such as math and linguistics. In more everyday language, a radical is someone who has very extreme views, so you could say that their views are different from the root up. Similarly, a radical flaw or change is a fundamental one whereas a radical design or idea is very new and innovative.

Primary Meanings of radical

(used of opinions and actions) far beyond the norm
a person who has radical ideas or opinions
a character conveying the lexical meaning of a logogram
of or relating to or constituting a linguistic root
an atom or group of atoms with at least one unpaired electron; in the body it is usually an oxygen molecule that has lost an electron and will stabilize itself by stealing an electron from a nearby molecule
(mathematics) a quantity expressed as the root of another quantity
arising from or going to the root or source
Full Definitions of radical

adj (used of opinions and actions) far beyond the norm

radical opinions on education”
extremist, ultra
beyond reasonable limits

adj markedly new or introducing radical change

radical political views”
not of long duration; having just (or relatively recently) come into being or been made or acquired or discovered

n a person who has radical ideas or opinions

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Mikhail Aleksandrovich Bakunin
Russian anarchist; ally and later opponent of Karl Marx (1814-1876)
Osama bin Laden
Arab terrorist who established al-Qaeda (born in 1957)
Marie Anne Charlotte Corday d'Armont
French revolutionary heroine (a Girondist) who assassinated Marat (1768-1793)
Georges Jacques Danton
French revolutionary leader who stormed the Paris bastille and who supported the execution of Louis XVI but was guillotined by Robespierre for his opposition to the Reign of Terror (1759-1794)
Emma Goldman
United States anarchist (born in Russia) who opposed conscription; was deported to the Soviet Union in 1919 (1869-1940)
Ernesto Guevara
an Argentine revolutionary leader who was Fidel Castro's chief lieutenant in the Cuban revolution; active in other Latin American countries; was captured and executed by the Bolivian army (1928-1967)
Jaffar Umar Thalib
Indonesian terrorist and Islamic militant who commands the Laskar Jihad; uses violence to achieve political ends (1965-2000)
Pyotr Alexeyevich Kropotkin
Russian anarchist (1842-1921)
Mao Tsetung
Chinese communist leader (1893-1976)
Jean Paul Marat
French revolutionary leader (born in Switzerland) who was a leader in overthrowing the Girondists and was stabbed to death in his bath by Charlotte Corday (1743-1793)
Jose Julian Marti
Cuban poet and revolutionary who fought for Cuban independence from Spain (1853-1895)
Karl Marx
founder of modern communism; wrote the Communist Manifesto with Engels in 1848; wrote Das Kapital in 1867 (1818-1883)
Honore-Gabriel Victor Riqueti
French revolutionary who was prominent in the early days of the French Revolution (1749-1791)
Ilich Ramirez Sanchez
Venezuelan master terrorist raised by a Marxist-Leninist father; trained and worked with many terrorist groups (born in 1949)
Maxmillien Marie Isidore de Robespierre
French revolutionary; leader of the Jacobins and architect of the Reign of Terror; was himself executed in a coup d'etat (1758-1794)
Nicola Sacco
United States anarchist (born in Italy) who with Bartolomeo Vanzetti was convicted of murder and in spite of world-wide protest was executed (1891-1927)
Lev Davidovich Bronstein
Russian revolutionary and Communist theorist who helped Lenin and built up the army; he was ousted from the Communist Party by Stalin and eventually assassinated in Mexico (1879-1940)
Benjamin Ricketson Tucker
United States anarchist influential before World War I (1854-1939)
Nat Turner
United States slave and insurrectionist who in 1831 led a rebellion of slaves in Virginia; he was captured and executed (1800-1831)
Bartolomeo Vanzetti
United States anarchist (born in Italy) who with Nicola Sacco was convicted of murder and in spite of world-wide protest was executed (1888-1927)
Denmark Vesey
United States freed slave and insurrectionist in South Carolina who was involved in planning an uprising of slaves and was hanged (1767-1822)
Francisco Villa
Mexican revolutionary leader (1877-1923)
Sir William Wallace
Scottish insurgent who led the resistance to Edward I; in 1297 he gained control of Scotland briefly until Edward invaded Scotland again and defeated Wallace and subsequently executed him (1270-1305)
Emiliano Zapata
Mexican revolutionary who led a revolt for agrarian reforms (1879-1919)
Chou En-lai
Chinese revolutionary and communist leader (1898-1976)
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anarchist, nihilist, syndicalist
an advocate of anarchism
Bolshevik, Marxist, bolshie, bolshy, red
emotionally charged terms used to refer to extreme radicals or revolutionaries
a person who holds extreme views
leveler, leveller
a radical who advocates the abolition of social distinctions
revolutionary, revolutionist, subversive, subverter
a radical supporter of political or social revolution
a radical who employs terror as a political weapon; usually organizes with other terrorists in small cells; often uses religion as a cover for terrorist activities
Trot, Trotskyist, Trotskyite
radicals who support Trotsky's theory that socialism must be established throughout the world by continuing revolution
a member of the Industrial Workers of the World
young Turk
a young radical who agitates for reform
counter-revolutionist, counterrevolutionary, counterrevolutionist
a revolutionary whose aim is to reverse the changes introduced by an earlier revolution
dynamiter, dynamitist
a person who uses dynamite in a revolutionary cause
Girondin, Girondist
a member of the moderate republican party that was in power during the French Revolution; the Girondists were overthrown by their more radical rivals the Jacobins
cyber-terrorist, cyberpunk, hacker
a programmer who breaks into computer systems in order to steal or change or destroy information as a form of cyber-terrorism
freedom fighter, insurgent, insurrectionist, rebel
a person who takes part in an armed rebellion against the constituted authority (especially in the hope of improving conditions)
a member of the radical movement that instituted the Reign of Terror during the French Revolution
a spy or saboteur or terrorist planted in an enemy country who lives there as a law-abiding citizen until activated by a prearranged signal
suicide bomber
a terrorist who blows himself up in order to kill or injure other people
Type of:
individual, mortal, person, somebody, someone, soul
a human being

n a character conveying the lexical meaning of a logogram

Type of:
character, grapheme, graphic symbol
a written symbol that is used to represent speech

n (linguistics) the form of a word after all affixes are removed

base, root, root word, stem, theme
Type of:
descriptor, form, signifier, word form
the phonological or orthographic sound or appearance of a word that can be used to describe or identify something

adj of or relating to or constituting a linguistic root

“a radical verb form”

n an atom or group of atoms with at least one unpaired electron; in the body it is usually an oxygen molecule that has lost an electron and will stabilize itself by stealing an electron from a nearby molecule

free radical
Type of:
(physics and chemistry) the smallest component of an element having the chemical properties of the element

n (chemistry) two or more atoms bound together as a single unit and forming part of a molecule

chemical group, group
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acyl, acyl group
any group or radical of the form RCO- where R is an organic group
alcohol group, alcohol radical
the chemical group -OH
aldehyde group, aldehyde radical
the chemical group -CHO
alkyl, alkyl group, alkyl radical
any of a series of univalent groups of the general formula CnH2n+1 derived from aliphatic hydrocarbons
allyl, allyl group, allyl radical
the univalent unsaturated organic radical C3H5; derived from propylene
amino, amino group
the radical -NH2
a hydrocarbon radical that occurs in many organic compounds
azido group, azido radical
the univalent group N3- derived from hydrazoic acid
azo group, azo radical
the bivalent group -N=N- united to two hydrocarbon groups
benzyl, benzyl group, benzyl radical
the univalent radical derived from toluene
benzoyl group, benzoyl radical
the univalent radical derived from benzoic acid
arsenic group, cacodyl, cacodyl group, cacodyl radical
the univalent group derived from arsine
carbonyl group
the bivalent radical CO
carboxyl, carboxyl group
the univalent radical -COOH; present in and characteristic of organic acids
the chemical group that gives color to a molecule
cyanide group, cyanide radical, cyano group, cyano radical
the monovalent group -CN in a chemical compound
a trivalent radical derived from glycerol by removing the three hydroxyl radicals
hydrazo group, hydrazo radical
the bivalent group -HNNH- derived from hydrazine
hydroxyl, hydroxyl group, hydroxyl radical
the monovalent group -OH in such compounds as bases and some acids and alcohols
ketone group
a group having the characteristic properties of ketones
methylene, methylene group, methylene radical
the bivalent radical CH2 derived from methane
propyl, propyl group, propyl radical
the monovalent organic group C3H7- obtained from propane
a hydrocarbon radical (C4H9)
nitro group
the group -NO3
the radical -NO2 or any compound containing it (such as a salt or ester of nitrous acid)
uranyl, uranyl group, uranyl radical
the bivalent radical UO2 which forms salts with acids
vinyl, vinyl group, vinyl radical
a univalent chemical radical derived from ethylene
acetyl, acetyl group, acetyl radical, ethanoyl group, ethanoyl radical
the organic group of acetic acid (CH3CO-)
ethyl, ethyl group, ethyl radical
the univalent hydrocarbon radical C2H5 derived from ethane by the removal of one hydrogen atom
an amino derivative of glucose that is a component of many polysaccharides
(chemistry) the negative univalent acyl radical CHO that occurs in aldehydes
methyl, methyl group, methyl radical
the univalent radical CH3- derived from methane
sodium nitrite
nitrite used to preserve and color food especially in meat and fish products; implicated in the formation of suspected carcinogens
Type of:
building block, unit
a single undivided natural thing occurring in the composition of something else

n (mathematics) a quantity expressed as the root of another quantity

Type of:
amount, measure, quantity
how much there is or how many there are of something that you can quantify

adj arising from or going to the root or source

“a radical flaw in the plan”
pertaining to or constituting a base or basis

adj especially of leaves; located at the base of a plant or stem; especially arising directly from the root or rootstock or a root-like stem

radical leaves”
especially of leaves; growing on a stem especially on the upper part of a stem

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