Fundamentalism is a tendency exhibited by significant minorities within most major religions. Fundamentalists seek to recapture a supposed ideological "purity," often by reasserting the primacy of their own idiosyncratic interpretation of religious texts over centuries of acquired knowledge and practice. This puts them at odds not only with the secular world and members of other religions, but also with their less zealous brethren.
There is an actual movement among English-speaking, especially American, Protestant Christians whose leaders sometimes describe as "fundamentalism." Founded at approximately the same historical moment as the First World War, the movement reflected the cultural disorientation of poorer, more rural, less educated Americans in a rapidly urbanizing America.