Radical

Latin radicalis — of or having roots. From radix — root. Originally a mathematical and grammatical term — the root form of a word or number. The political sense — change at the root level — emerged in the 18th century, especially with the British Radical movement advocating for parliamentary reform.

Political 'radical' historically applied to advocates of fundamental change — from the left (radical reformers, radical Republicans during Reconstruction) and the right (radical traditionalists). 'Radicalization' is now most often applied to extremist religious or political violence. The original meaning — going to the root — survives as a term of approval in some intellectual contexts.