The Associated Press Stylebook has a decent working definition of Fundamentalism (even if most reporters don’t heed it):

fundamentalist The word gained usage in an early 20th century fundamentalist-modernist controversy within Protestantism. In recent years, however, fundamentalist has to a large extent taken on pejorative connotations except when applied to groups that stress strict, literal interpretations of Scripture and separation from other Christians. In general, do not use fundamentalist unless a group applies the word to itself.