Islamism (also often called political Islam or Islamic fundamentalism) is a religio-political ideology. There is no consensus definition of Islamism, which has many varieties and alternative names, and some have objected to use of the term, either for its being derogatory, or so broad and flexible as to have lost its meaning. In its original formulation, Islamism described an ideology seeking to revive Islam to its past assertiveness and glory, purifying it of foreign elements, reasserting its role into “social and political as well as personal life"; and in particular “reordering government and society in accordance with laws prescribed by Islam" (aka Sharia). According to at least one observer (author Robin Wright), Islamist movements have "arguably altered the Middle East more than any trend since the modern states gained independence", redefining "politics and even borders".

Central and prominent figures in 20th-century Islamism include Sayyid Rashid Rida, Hassan al-Banna (founder of the Muslim Brotherhood), Sayyid Qutb, Abul A'la Maududi, Ruhollah Khomeini (founder of the Islamic Republic of Iran), Hassan Al-Turabi.

Al-Banna and Maududi called for a "reformist" strategy to re-Islamizing society through grassroots social and political activism. Other Islamists (Al-Turabi) have advocated a "revolutionary" strategy of Islamizing society through exercise of state power, or (Sayyid Qutb) for combining grassroots Islamization with violent revolution. The term has been applied to non-state reform movements, political parties, militias and revolutionary groups. Islamists emphasize the implementation of sharia, pan-Islamic political unity, the creation of Islamic states, (eventually unified), and rejection of non-Muslim influences—particularly Western or universal economic, military, political, social, or cultural.

At least one author (Graham E. Fuller) has argued for a broader notion of Islamism as a form of identity politics, involving "support for [Muslim] identity, authenticity, broader regionalism, revivalism, [and] revitalization of the community." Islamists themselves prefer terms such as "Islamic movement", or "Islamic activism" to "Islamism", objecting to the insinuation that Islamism is anything other than Islam renewed and revived. In public and academic contexts, the term "Islamism" has been criticized as having been given connotations of violence, extremism, and violations of human rights, by the Western mass media, leading to Islamophobia and stereotyping.

Following the Arab Spring, many post-Islamist currents became heavily involved in democratic politics, while others spawned "the most aggressive and ambitious Islamist militia" to date, such as the Islamic State of Iraq and the Levant (ISIL).