Kafir (كافر DIN; plural كَافِرُونَ DIN, كفّار DIN or كَفَرَة DIN; feminine كافرة DIN; feminine plural كافرات DIN or كوافر DIN) is an Arabic and Islamic term which, in the Islamic tradition, refers to a person who disbelieves in God as per Islam, denies his authority, or rejects the tenets of Islam; or simply all those who are not Muslims—those who do not believe in the guidance of Islamic prophet Muhammad. Thus the term DIN is the opposite of the term Muslim. The term is often translated as 'infidel,' 'pagan,' 'rejector,' 'denier,' 'disbeliever,' 'unbeliever,' 'nonbeliever,' and 'non-Muslim.' The term is used in different ways in the Quran, with the most fundamental sense being 'ungrateful' (toward God). Kufr means 'unbelief' or 'non-belief,' 'to be thankless,' 'to be faithless,' or 'ingratitude.' The opposite term of kufr is īmān ('faith'), and the opposite of kāfir is muʾmin ('believer'). A person who denies the existence of a creator might be called a dahri.
Kafir is sometimes used interchangeably with mushrik (مشرك, those who practice polytheism), another type of religious wrongdoer mentioned frequently in the Quran and other Islamic works. (Other, sometimes overlapping Quranic terms for wrong-doers are ẓallām ('villain', 'oppressor') and fāsiq ('sinner', 'fornicator').) Historically, while Islamic scholars agreed that a polytheist/mushrik was a kafir, they sometimes disagreed on the propriety of applying the term to Muslims who committed a grave sin or to the People of the Book. The Quran distinguishes between mushrikun and People of the Book, reserving the former term for idol-worshippers, although some classical commentators considered the Christian doctrine to be a form of shirk.
In modern times, kafir is sometimes applied towards self-professed Muslims particularly by members of Islamist movements. The act of declaring another self-professed Muslim a kafir is known as takfir, a practice that has been condemned but also employed in theological and political polemics over the centuries.