For the last couple of centuries, the word ‘‘radical’’ referred to people who try to pull things up from the root. After the Civil War, Radical Republicans, as they were called, pushed Reconstruction policies through Congress on behalf of freed slaves, trying to tear down the plantation-based power structure of the antebellum South. Eugene Debs, the most prominent socialist of the early 20th century (and a hero to Bernie Sanders), ran for president five times on what was seen as a radical platform of condemning capitalist oppression.

Change that is ‘‘radical’’ is ‘‘essential and fundamental’’ or ‘‘far-reaching,’’ according to the Oxford English Dictionary. About 50 years ago, surfers pressed ‘‘radical’’ into service to describe extreme waves. It became generic slang for anything remarkable or worthy of attention, sometimes shortened to ‘‘rad’’ and often paired with ‘‘totally.’’ In its other meanings, ‘‘radical’’ has retained its potency as well as a hint of the uncontrollable. (In chemistry, ‘‘free radicals’’ are especially jittery atoms containing unpaired electrons.) ‘‘Radical chic’’ was the mocking phrase Tom Wolfe coined in New York magazine in 1970 for the celebrity feting of militants. Reeling off a list of attendees at a party in Leonard Bernstein’s Manhattan penthouse, he built to the crescendo ‘‘. . . and now, in the season of Radical Chic, the Black Panthers.’’

Right now we’re most likely to see ‘‘radical’’ paired with ‘‘Islam’’ in news stories and conversations about the motivations of the San Bernardino shooters, Tashfeen Malik and Syed Rizwan Farook. Yet many Muslims reject the term ‘‘radical Islam.’’ They say that ISIS’s reading of the Quran and other texts is so selective as to be unrecognizable as Muslim at all. Dar al-Ifta, an authority on Islamic law in Egypt, asked the media and others to stop referring to ISIS as ‘‘the Islamic State’’ and instead use the name QSIS, for Qaeda Separatists in Iraq and Syria.

President Obama and Hillary Clinton live in the world of politics, where rhetoric is often more heated, but they avoid using ‘‘radical Islam’’ or ‘‘jihad’’ to describe the terror driven by ISIS. After the group took credit for the attacks in Paris that killed 130 people last month, Obama said that Muslims around the world were our allies and encouraged them ‘‘to ask very serious questions about how did these extremist ideologies take root, even if it’s only affecting a very small fraction of the population.’’ He and Clinton have warned that defining ISIS in terms of Islam — even with ‘‘radical’’ attached — risks alienating Muslims and handing ISIS a recruitment tool. ‘‘It helps to create this clash of civilizations,’’ Clinton said the weekend after the San Bernardino shootings.

But Republican candidates for president charge Obama and Clinton with making a dangerous denial in linguistic form. ‘‘Look, we are having a tremendous problem with radical Islamic terrorism,’’ Donald Trump saidon ‘‘Face the Nation’’ after the San Bernardino shootings. ‘‘And we have a president that won’t issue the term. He won’t talk about it.’’ Gov. Chris Christie of New Jersey made the same point on the same show: ‘‘They won’t say radical Islamic jihadist.’’ The implied question was, How can the country fight its enemies if the president won’t say who they are? ...