Fundamentalism is a tendency among certain groups and individuals that is characterized by the application of a strict literal interpretation to scriptures, dogmas, or ideologies, along with a strong belief in the importance of distinguishing one's ingroup and outgroup, which leads to an emphasis on some conception of "purity", and a desire to return to a previous ideal from which advocates believe members have strayed. The term is usually used in the context of religion to indicate an unwavering attachment to a set of irreducible beliefs (the "fundamentals").

The term "fundamentalism" is generally regarded by scholars of religion as referring to a largely modern religious phenomenon which, while itself a reinterpretation of religion as defined by the parameters of modernism, reifies religion in reaction against modernist, liberal and ecumenical tendencies developing in religion and society in general that it perceives to be foreign to a particular religious tradition. Depending upon the context, the label "fundamentalism" can be a pejorative rather than a neutral characterization, similar to the ways that calling political perspectives "right-wing" or "left-wing" can have negative connotations.