The word Horizon أفق mentioned 03 times in Quran in 03 verses. 

(41:53)

سَنُرِيهِمْ آيَاتِنَا فِي الْآفَاقِ وَفِي أَنْفُسِهِمْ

We will show them Our signs in the horizons and within themselves until it becomes clear to them that it is the truth. But is it not sufficient concerning your Lord that He is, over all things, a Witness?

(53:7)

وَهُوَ بِالْأُفُقِ الْأَعْلَىٰ

While he was in the higher [part of the] horizon.

(81:23)

وَلَقَدْ رَآهُ بِالْأُفُقِ الْمُبِينِ

And he has already seen Gabriel in the clear horizon.