rad·i·cal 
(rădĭ-kəl)
(rădĭ-kəl)adj.
1. Arising from or going to a root or source; basic: proposed a radical solution to the problem.
2. Departing markedly from the usual or customary; extreme or drastic: a radical change in diet.
3. Relating to or advocating fundamental or revolutionary changes in current practices, conditions, or institutions: radical politics; a radical political theorist.
4. Medicine Relating to or being surgery that is extreme or drastic in an effort to eradicate all existing or potential disease: radical hysterectomy.
5. Linguistics Of or being a root: a radical form.
6. Botany
a. Of, relating to, or arising from a root: radical hairs.
b. Arising from the base of a stem or from a below-ground stem or rhizome: radical leaves.
7. Slang Excellent; wonderful.
n.
1. One who advocates fundamental or revolutionary changes in current practices, conditions, or institutions: radicals seeking to overthrow the social order.
2. Mathematics The root of a quantity as indicated by the radical sign.
3. Symbol R An atom or a group of atoms with one unpaired electron.
4. Linguistics See root1.
5.
a. Any of the basic Chinese characters that are combined to form more complex characters.
b. Any of the traditional set of basic strokes or groups of strokes that make up Chinese characters and are used to classify and organize them in dictionaries.
[Middle English, of a root, from Late Latin rādīcālis, having roots, from Latin rādīx, rādīc-, root; see wrād- in the Appendix of Indo-European roots.]
radi·cal·ly adv.
radi·cal·ness n.