Fundamentalism is an term used to describe religious movements that reject modernism (particularly scientific thought and secularism), that claim to find all truth, including science, history, and psychology, in their sectarian scriptures, and that often try to impose some degree of religious law on all citizens of any government they are able to control. The type species is the Christian Fundamentalism of the 1920s, which is a reaction against Darwin's theory of evolution; by extension the term is applied to Haredi Judaism and Wahhabism in Islam. In more abstract terms, it can refer to any movement to recapture an ideological "purity" within a religion, that supposedly has been lost by mainstream adherents of the religion at large. Note that this definition would also apply to liberal Christians who seek a Sermon-on-the-Mount purity by engaging in works of charity. Fundamentalists often assert the primacy of their own idiosyncratic interpretation of religious texts over centuries of acquired knowledge and practice. This has the advantage of making great insights and profound wisdom available to all believers, but puts them at odds not only with the secular world and members of other religions, but also with their own brethren who find meaning in the modern world.
Though the term has been applied to anyone within any religion desiring to go back to the so-called fundamentals of the religion, the term originally applied to a more formal movement within Protestant Christianity in the 1920s that emphasized a literal reading of the Bible and a more militant approach to Christianity. The formal movement (often represented with a capital F) ran out of steam in the mid-1930s, but the emphasis on literalism, inerrancy, and exceptionalism have permeated many different denominations within Christianity, giving rise to local churches within the larger body, who self-identify as "fundamental churches".