المعنى الاصطلاحي :

Perfect knowledge of Tawhīd (Islamic monotheism), without having any doubt, distrust, and hesitation.

الشرح المختصر :

"Yaqeen" (certitude) refers to high-level and categorical knowledge and belief, which means firm and absolute conviction that accords with doubtless reality. A "mūqin" is a person with knowledge that contains no doubt. If he knows that "there is no god but Allah" means that there is no deity worthy of worship except Allah, his knowledge should not be subject to any doubt, and he should not feel hesitant in faith in terms of the wording and meaning of this statement. He should also be certain that this statement is true: that Allah Almighty is the true God, and that all others are false gods and none of them has the right to be worshiped in any way. Among the proofs for this condition is the verse that reads: {The believers are only the ones who have believed in Allah and His Messenger and then did not doubt, but strove with their properties and their lives in the cause of Allah. It is those who are the truthful.} [Al-Hujurāt: 15]

التعريف اللغوي المختصر :

"Yaqeen": asserting and affirming decisively. Opposite: "shakk", "rayb" (doubt). Original meaning: the state of being calm and still, or of becoming apparent and clear.