Imam Anwar al-Awlaki says: Yaqeen is certainty in your beliefs over your physical reality. Your heart soaks up the belief and becomes unshakable.
The Prophet (صلي الله عليه وسلم) had it. All four khulafa had it.
O Allah, grant us belief in the hereafter and make our hearts firm.
Maybe a future article about the different types of Yaqeen? i.e. Aynul Yaqeen, Ilmul Yaqeem, and so on… ;) See Surah Takathur for some hints