From radical +‎ -ism.

• IPA^(key): /ˈradɪkəlɪz(ə)m/

• Audio (Southern England):

  (file)

radicalism (countable and uncountable, plural radicalisms)

1. Any of various radical social or political movements that aim at fundamental change in the structure of society

• radicalist

Borrowed from French radicalisme. By surface analysis, radical +‎ -ism.

radicalism n (uncountable)

1. radicalism