تكفير
takfīr
Declaring another Muslim to be an unbeliever, which classical scholars treated with extreme caution.
What it means
The Prophet said: “If a man says to his brother, ‘O unbeliever’, it returns upon one of them” (Sahih al-Bukhari). Mainstream scholars held that a person who pronounces the declaration of faith is not to be called an unbeliever unless there is clear proof of rejecting a certain, essential part of the faith, and only a court of qualified judges may rule.
What the sources say
Extremist groups have used takfir to justify violence against Muslims, and the great majority of scholars have condemned that usage.