The term “radical” is generally applied to groups and individuals seeking to transform or destroy part or all of the existing social order. The first appearance of the word in a political context occurred in England in 1797, when Charles James Fox, Britain's first foreign secretary, called for a “radical reform” that would extend to all his countrymen the right to vote in general elections. The term thereafter was used to identify anyone who similarly supported the movement for parliamentary reform, meaning the expansion of voting privileges (traditionally reserved for upper levels of property holders) to less-wealthy and broader segments of the working-class population. When the Reform Act of 1867 widened the right of suffrage in this manner, the English Radicals spearheaded efforts to organize the newly eligible voters, and helped transform the Whig parliamentary faction into the Liberal Party.

By today's standards, 19th-century English Radicals bore much greater resemblance to gradualist reformers than to unbridled revolutionaries. They were influenced by philosophical tenets which held that people are able to control and alter their social environment by means of collective action. But because this assumption also underlay communist theories of social reform, the label “radical” became, over time, largely synonymous with communists and other advocates of violent social change. As the term's meaning thus evolved, it ceased to evoke images of gradualist reformers.

Today radicalism, like communism, is marked by an overriding desire to dramatically transform the existing society and its institutions by any means necessary, on the theory that the society is decadent to its core and possesses no redeeming features that are worth defending.

But not all radicals are willing to publicly declare their extremist agendas. Indeed in recent decades many radicals have chosen to eschew calls for open defiance and violent revolution, in favor of stealth, or "Trojan Horse," strategies. Beginning in the 1970s the radicals of the New Left adopted the tactics of the infamous Saul Alinsky, working to change society incrementally by first infiltrating its major institutions – the schools, the media, the churches, the entertainment industry, the labor unions, and the three branches of government – and then using their newfound influence to implement policies from those seats of power. Most notably, these ex-New Left radicals found a home in the Democratic Party, where their influence has steadily grown over time. As such, radicalism has made its way into the mainstream of American political life.