adj. & n.
adj. 1 of the root or roots; fundamental (a radical error). 2 far-reaching; thorough; going to the root (radical change). 3 a advocating thorough reform; holding extreme political views; revolutionary. b (of a measure etc.) advanced by or according to principles of this kind. 4 forming the basis; primary (the radical idea). 5 Math. of the root of a number or quantity. 6 (of surgery etc.) seeking to ensure the removal of all diseased tissue. 7 of the roots of words. 8 Mus. belonging to the root of a chord. 9 Bot. of, or springing direct from, the root. 10 hist. belonging to an extreme section of the Liberal party. 11 US hist. seeking extreme anti-South action at the time of the Civil War.
n. 1 a person holding radical views or belonging to a radical party. 2 Chem. a a free radical. b an element or atom or a group of these normally forming part of a compound and remaining unaltered during the compound's ordinary chemical changes. 3 the root of a word. 4 a fundamental principle; a basis. 5 Math. a a quantity forming or expressed as the root of another. b a radical sign. radical sign indicating the square, cube, etc., root of the number following. radicalism n. radicalize v.tr. & intr. (also -ise). radicalization n. radically adv. radicalness n.
[ ME f. LL radicalis f. L radix radicis root ]