What is the meaning of disbelief (kufr) and who is a disbeliever (kafir)?

Linguistically, kufr (disbelief) means covering and concealing. Terminologically, it is what contradicts faith, whether in words, actions, or beliefs. Disbelief may be an inner denial in the heart, or a heart-bound action such as hating Allah and His Messenger, or a verbal statement such as insulting Allah and mocking His religion, or an outward action such as prostrating to an idol.

As for the kafir (disbeliever), it is one in whom kufr is found. It is necessary to differentiate between attributing kufr to an action and designating kufr to a specific person. Not everyone who falls into kufr has kufr applied to them, as takfir (declaring someone a disbeliever) has conditions and impediments that may be absent in the case of a specific individual. Islam is not removed from a Muslim until after the establishment of proof and the removal of ambiguity.