Kufr

Disbelief, or ingratitude: literally “covering” the truth, in the heart or in deed.

What it means

“Kafara” means to cover, as a farmer covers seed, and the Quran uses it of rejecting the truth and also of being ungrateful for God’s favours (14:7, 27:40). Kufr in this sense can be of belief or of behaviour. It is not for people to decide in whose heart faith lies: the Prophet warned that whoever calls his brother a disbeliever, it returns on one of them (Sahih al-Bukhari).

Term and Arabic as listed in Ismail al Faruqi, Toward Islamic English (International Institute of Islamic Thought, 1995), p. 45, listed as KUFR. The definition is restated here.