ex ‧ trem ‧ ist /ɪkˈstriːməst, ɪkˈstriːmɪst/ BrE AmE noun [countable]
[ Word Family: noun : ↑ extreme , ↑ extremism , ↑ extremist , ↑ extremity ; adjective : ↑ extreme , ↑ extremist ; adverb : ↑ extremely ]
someone who has extreme political opinions and aims, and who is willing to do unusual or illegal things in order to achieve them:
The bomb was planted by right-wing extremists.
—extremist adjective