A reader writes in to complain that he is having, well, a reaction to the use of the word "radical." He was taught as an undergraduate some decades ago, he says, that "radical" was a pejorative term for the political far left. "Reactionary" was the counterpart to refer to the political far right.

But now radical seems to have been downgraded to an all-purpose synonym for "extreme."

"Please help," he writes, "before I 'react' in the extreme!"

Dear Reader, you might say that radical has returned to its roots. Its root actually is root, in fact. Our English word derives from the Latin radix, which means simply "root," and you may recall working with the radical sign in math class to represent square, cube, and other "roots."

Radish derives from that same Latin word. One of my college professors, attempting to walk the class through the French political system, referred to the Radical Party. Its members evidently liked to let on that they were further to the left than was actually the case. They were like the radish, he said – red (socialist or even communist) on the outside, but white (conservative) on the inside.

Radical politicians, or religious believers, for that matter, are arguably those seeking to return, or claiming to return, to the roots of their ideology or faith – fundamentalists, we might also call them. When radical first came into the language in the 1650s, it meant, according to the Online Etymology Dictionary, "going to the origin, essential." By the time it made its way into political discourse a century and a half later, though, it carried the idea of "change from the roots."

The pairing of "radical" and "extreme" is interesting in light of the English idiom (less often used in the United States) "root and branch." It means "thorough" and is often used in calls for "root and branch reform" of schools, prisons, and the like. In a negative sense, "root and branch" means "utterly," as in "destroyed root and branch."

Extreme, meanwhile, comes from a Latin word meaning "outermost, utmost, farthest," and so corresponds roughly to the branches of a tree. If political "radicals" have gone back to first principles (which is redundant if you know that principle comes from the Latin for "first"), political "extremists" have etymologically taken things "to the outer limit" or "to the far boundary." The two terms are based on two different metaphors, but in practice, either is likely to be applied by those on either side of a divide to their opponents – to conservatives deemed too conservative, or to leftists deemed too leftist.

All that said, I would not dispute Dear Reader's account of how these adjectives have traditionally been applied. The meanings of words are like roads that may widen or narrow as they stretch out across the countryside. The first "Radical Republicans" were the ones who thought Abraham Lincoln was too soft on slavery.

To frame the terms of a debate is to start to win the debate. The Marxist terminology was "revolutionary" for those on the left and "reactionary" for those on the right. The Marxists themselves clearly thought "revolutionary" was the side to be on. Their ideology has been discredited, but one can argue that they won at least a small moral victory by controlling the terms of the debate. After all, would you rather be seen as a "revolutionary" or a "reactionary"?