Pronunciation: (rad'i-kul),[key] —adj. 1. of or going to the root or origin; fundamental: a radical difference. 2. thoroughgoing or extreme, esp. 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. Math. a. pertaining to or forming a root.
b. denoting or pertaining to the radical sign.
c. irrational (def. 5b).
7. Gram.of or pertaining to a root.
8. Bot.of or arising from the root or the base of the stem.
—n. 1. a person who holds or follows strong convictions or extreme principles; extremist.
2. a person who advocates fundamental political, economic, and social reforms by direct and often uncompromising methods.
3. Math. a. a quantity expressed as a root of another quantity.
b. the set of elements of a ring, some power of which is contained in a given ideal.
c. See radical sign. 4. Chem. a. group (def. 3).
b. See free radical. 5. Gram.root (def. 11).
6. (in Chinese writing) one of 214 ideographic elements used in combination with phonetics to form thousands of different characters.