Islamic fundamentalism (Arabic: usul, the "fundamentals") is a term used to describe religious ideologies seen as advocating a return to the "fundamentals" of Islam: the Quran and the Sunnah. Definitions of the term vary. According to Christine L. Kettel, it is deemed problematic by those who suggest that Islamic belief requires all Muslims to be fundamentalists, and by others as a term used by outsiders to describe perceived trends within Islam. Exemplary figures of Islamic fundamentalism who are also termed Islamists are Sayyid Qutb, Ruhollah Khomeini, Abul Ala Mawdudi, and Israr Ahmad. The 1979 Islamic Revolution in Iran is seen by Western scholars as a political success of Islamic fundamentalism. Economist Eli Berman argues that Radical Islam is a better term for many post-1920s movements starting with the Muslim Brotherhood, because these movements are seen to practice "unprecedented extremism", thus not qualifying as return to historic fundamentals.

According to academic John Esposito, one of the most defining features of Islamic fundamentalism is belief in the "reopening" of the gates of ijtihad.Graham Fuller describes Islamic fundamentalism not as distinct from Islamism but as a subset, "the most conservative element among Islamists." Its "strictest form" includes "Wahhabism, sometimes also referred to as salafiyya. ... For fundamentalists the law is the most essential component of Islam, leading to an overwhelming emphasis upon jurisprudence, usually narrowly conceived." Another American observer, Robert Pelletreau, Jr., assistant secretary of state for Near Eastern affairs, believes it the other way around, Islamism being the subset of Muslims "with political goals ... within" the "broader fundamentalist revival". Still another, Martin Kramer, sees little difference between the two terms: "To all intents and purposes, Islamic fundamentalism and Islamism have become synonyms in contemporary American usage."