extreme
/ɪkˈstriːm/
adjective
1.
being of a high or of the highest degree or intensity: extreme cold, extreme difficulty
2.
exceeding what is usual or reasonable; immoderate: extreme behaviour
3.
very strict, rigid, or severe; drastic: an extreme measure

radical
/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
short for free radical
another name for group (sense 10)
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