Kafir is an Arabic term in Islam which refers to a person who disbelieves the [God], denies his authority, who reject the truth.
Kafir is often translated as 'infidel truth denier', 'rejector', 'disbeliever', 'unbeliever'. The term is used in different ways in the Quran, with the most fundamental sense being ungrateful toward God. Kufr means 'disbelief', 'unbelief', 'non-belief', 'to be thankless', 'to be faithless', or 'ingratitude'. The opposite term of kufr is iman, and the opposite of kafir is mu'min. A person who denies the existence of a creator might be called a dahri. From Wikipedia
