«وَالْيَقِينُ: هُوَ الِاعْتِقَادُ الْجَازِمُ الْمُطَابِقُ»
Certainty is firm conviction corresponding to reality.
Jazim means decisive rather than hesitant; mutabiq means that the conviction accords with the facts. Strong psychological confidence alone is not sufficient for certainty in this technical sense.
Illustration: Someone may feel absolutely sure that an event happened today when established evidence shows it happened yesterday. The conviction may be unwavering, but it is not corresponding to reality, and therefore does not satisfy the definition.