المعنى الاصطلاحي :

A statement, act, belief, doubt, or neglect of an obligation which takes one out of the fold of Islam.

الشرح المختصر :

"Kufr" (disbelief) is classified into two categories: 1. Major "kufr": it refers to all statements, actions, or beliefs that take one out of the fold of Islam. It could also be a wrong conviction harbored in one's heart or an act of the heart, such as detesting Allah, His words or Messenger, may Allah's peace and blessings be upon him. It could also be a blasphemous statement, like insulting Allah, or a blasphemous act, like prostrating oneself before an idol or offering animal sacrifices to other than Allah. There are many types of major "kufr": a) "Kufr al-Juhood wat-Takdheeb" (disbelief of rejection): this form of "kufr" could be a conviction harbored in one's heart, verbalized by the tongue, or translated into action by the body parts. It applies to someone who acknowledges the truth in his heart but rejects it with his tongue (or action). This type of "kufr" is applicable to the Jews, who disbelieved in the Prophet, may Allah's peace and blessings be upon him, out of spite and arrogance (despite their knowledge of his truthfulness). b) "Kufr al-Istikbār" (disbelief of arrogance): which is rejecting the truth and refraining from seeking it or acting upon it, be it a statement, an action, or a belief, such as the disbelief of the devil. c) "Kufr an-Nifāq" (disbelief of hypocrisy): this means a lack of the conviction harbored in the heart and reflected accordingly in the actions of the heart despite the apparent compliance. d) "Kufr ash-Shakk wa ar-Reebah" (disbelief of doubt and hesitation): this applies to the person's hesitation to adhere to the truth or doubting it. It is required of the Muslim to believe with certainty about that which the Prophet, may Allah's peace and blessings be upon him, was sent with, without any slight doubt or hesitation; he should believe in it by heart with a firm conviction and unshakable faith. 2. Minor "kufr": this category of "kufr" incorporates the sins and misdeeds that are declared as acts of disbelief as per the Shariah yet do not take the doer out of the fold of Islam nor amount to major "kufr". This type of "kufr" includes the person's ungratefulness towards Allah for His blessings and the acts that are described as "kufr doona kufr" (lesser form of "kufr", i.e. minor disbelief), such as fighting one's fellow Muslims, swearing by anything other than Allah, and wailing over the dead.

التعريف اللغوي المختصر :

"Kufr": Opposite: "imān" (faith). Original meaning: concealment and coverage; the non-Muslim is named as "kāfir" (disbeliever) because he has concealed the truth and faith with disbelief. Other meanings: denial and ungratefulness.