1. adjective /ˈɹædɪkəl/
a) Favouring fundamental change, or change at the root cause of a matter.
His beliefs are radical.
b) Of or pertaining to a root .
The spread of the cancer required radical surgery, and the entire organ was removed.
See Also: radix
2. noun /ˈɹædɪkəl/
a) A member of the most progressive wing of the Liberal Party; someone favouring social reform (but generally stopping short of socialism).
b) A member of an influential, centrist political party favouring moderate social reform, a republican constitution, and secular politics.
Wikipedia foundation.