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[rad-i-kuh l] /ˈræd ɪ kəl/

adjective
1.
of or going to the root or origin; fundamental:
a radical difference.
2.
thoroughgoing or extreme, especially as regards change from accepted or traditional forms:
a radical change in the policy of a company.
3.
favoring drastic political, economic, or social reforms:
radical ideas; radical and anarchistic ideologues.
4.
forming a basis or foundation.
5.
existing inherently in a thing or person:
radical defects of character.
6.
Mathematics.

7.
Grammar. of or relating to a root.
8.
Botany. of or arising from the root or the base of the stem.
noun
9.
a person who holds or follows strong convictions or extreme principles; extremist.
10.
a person who advocates fundamental political, economic, and social reforms by direct and often uncompromising methods.
11.
Mathematics.

12.
Chemistry.

13.
Grammar. 1 (def 12).
14.
(in Chinese writing) one of 214 ideographic elements used in combination with phonetics to form thousands of different characters.
/ˈrædɪkəl/
adjective
1.
of, relating to, or characteristic of the basic or inherent constitution of a person or thing; fundamental: a radical fault
2.
concerned with or tending to concentrate on fundamental aspects of a matter; searching or thoroughgoing: radical thought, a radical re-examination
3.
favouring or tending to produce extreme or fundamental changes in political, economic, or social conditions, institutions, habits of mind, etc: a radical party
4.
(med) (of treatment) aimed at removing the source of a disease: radical surgery
5.
(slang, mainly US) very good; excellent
6.
of, relating to, or arising from the root or the base of the stem of a plant: radical leaves
7.
(maths) of, relating to, or containing roots of numbers or quantities
8.
(linguistics) of or relating to the root of a word
noun
9.
a person who favours extreme or fundamental change in existing institutions or in political, social, or economic conditions
10.
(maths) a root of a number or quantity, such as ³√5, √x
11.
(chem) Also radicle

12.
(linguistics) another word for root1 (sense 9)
13.
(in logographic writing systems such as that used for Chinese) a part of a character conveying lexical meaning
adj.

late 14c., in a medieval philosophical sense, from Late Latin radicalis “of or having roots,” from Latin radix (genitive radicis) “root” (see radish). Meaning “going to the origin, essential” is from 1650s. Radical sign in mathematics is from 1680s.

Political sense of “reformist” (via notion of “change from the roots”) is first recorded 1802 (n.), 1817 (adj.), of the extreme section of the British Liberal party (radical reform had been a current phrase since 1786); meaning “unconventional” is from 1921. U.S. youth slang use is from 1983, from 1970s surfer slang meaning “at the limits of control.” Radical chic is attested from 1970; popularized, if not coined, by Tom Wolfe. Radical empiricism coined 1897 by William James (see empiricism).
n.

1630s, “root part of a word, from radical (adj.) Political sense from 1802; chemical sense from 1816.

radical rad·i·cal (rād’ĭ-kəl)
n.
Abbr. R

adj.

radical
(rād’ĭ-kəl)

In politics, someone who demands substantial or extreme changes in the existing system.

In chemistry, an atom or group of atoms that has at least one electron free to participate in forming a chemical bond.

Note: In general, radicals are associated with chemical reactions that proceed rapidly.