certainty / absolute conviction
Masculine noun. Related: مُوقِنٌ (one who has certainty), أَيْقَنَ (to be certain). Three levels in the Quran: عِلْمُ الْيَقِينِ (knowledge of certainty), عَيْنُ الْيَقِينِ (eye of certainty), حَقُّ الْيَقِينِ (truth of certainty).
Al-yaqīn here is interpreted by most scholars as death - meaning one should worship God throughout life until death. The three levels of yaqīn represent ascending stages of spiritual realization.