Kafir is an Arabic word meaning “nonbeliever” or “infidel” (plural: kuffar). The term kafir derives from the Islamic word kufr (disbelief), used to describe those who deny Allah’s authority or teachings. In some interpretations of Islam, a kafir is considered the second stage of rebellion against God, the first being “fisq” and the third being “taghut.” According to Islamic theology, kufr has several sub-categories, including disbelief stemming from stubbornness (Kufr ul-‘Inad), denial (Kufr ul-Inkar), and hypocrisy (Kufr ul-Nifaq). 

Muslims who refer to self-described Muslims as kuffar engage in a highly controversial practice known as takfir. By designating another Muslim as a kafir, the accusing party partakes in an excommunication process that carries with it a de facto death sentence. In the 20th century, extremist ideologue Sayyid Qutb revived the medieval practice of takfir to label Muslim government leaders and Western political adversaries as kafir, in essence calling for their deaths. 

Supporters of ISIS, al-Qaeda, and other Islamic extremist groups use variants of the words “kafir” and “kuffar” to refer to their Muslim and non-Muslim adversaries in online forum, as well as on social media and through various propaganda materials. Extremist recruiters and propagandists have also sweepingly referred to Muslims and non-Muslims as kuffar.