Kufr is an Arabic and Islamic term that 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 term is often translated as infidel, pagan, rejector, denier, disbeliever, unbeliever, nonbeliever, and non-Muslim. 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) .

In Shar`i terminology, kufr means “not believing in Allah and His Messenger, whether that is accompanied by denial or it is not accompanied by denial but rather doubt, or turning away from faith out of jealousy or arrogance, or because one is following whims and desires that prevent one from following the message". The scholars divided kufr into a number of categories, under which they listed many forms and kinds of shirk. These are as follows:

• Kufr ul-Inkar: Disbelief out of denial. This applies to someone who denies with both heart and tongue.
• Kufr ul-Kibr: Disbelief out of arrogance and pride.
• Kufr ul-Juhud: Disbelief out of rejection. This applies to someone who acknowledges the truth in his heart but rejects it with his tongue.

The term kufr is also used to refer to transgression which is far short of a total rejection of Islam and does not amount to a rejection and denial of Allah, Glory be to Him, and His Messenger. The well-known Muslim scholar Imam Ibn Al-Qayyim divided kufr into two categories, namely major kufr and minor kufr. Major kufr leads to eternal punishment in the Hellfire while minor kufr leads to temporary rather than eternal punishment in the Hellfire.

In summary, kufr is a term used in Islamic culture to refer to a person who disbelieves in God as per Islam, denies his authority, or rejects the tenets of Islam. It means unbelief or non-belief, to be thankless, to be faithless, or ingratitude. The term is used in different ways in the Quran, with the most fundamental sense being ungrateful (toward God). The scholars divided kufr into a number of categories, under which they listed many forms and kinds of shirk.