Fundamentalism is the demand for a strict adherence to specific theological doctrines usually understood as a reaction against Modernist theology, combined with a vigorous attack on outside threats to their religious culture. The term "fundamentalism" was originally coined by its supporters to describe a specific package of theological beliefs that developed into a movement within the Protestant community of the United States in the early part of the 20th century, and that had its roots in the Fundamentalist–Modernist Controversy of that time. The term usually has a religious connotation indicating unwavering attachment to a set of irreducible beliefs. "Fundamentalism" is sometimes used as a pejorative term, particularly when combined with other epithets (as in the phrase "right-wing fundamentalists").

Evangelicals share much the same theology as Fundamentalists but put much less emphasis on the aggressive defense of that theology.^{[citation needed]}

Fundamentalism as a movement arose in the United States, starting among conservative Presbyterian theologians at Princeton Theological Seminary in the late 19th century. It soon spread to conservatives among the Baptists and other denominations around 1910-1920. The movement's purpose was to reaffirm key theological tenets and defend them against the challenges of liberal theology and higher criticism.