Actually the definition is more nuanced. A radical (from latin radix = root) is someone who wants to change the political system from its core by using legal procedures like elections. An extremist is someone who despite being in a minority trys to overthrow the system in violent terms. So depending on Trump’s win or a mop running in the capitol and declaring Trump the winner, they are either radicals or extremist.
Actually this is the result of strong and convincing left parties and the fear of conservatives that communism might get more approval. At least in Germany.