The one who is aware of everything. The one who hears every voice in existence - the loud and quiet. The one who sees everything in existence - the insignificant and significant, the small and large. The one whose knowledge encompasses everything. The one who will testify for or against His servants for what they did.
“Say: What thing is the greatest witness? Say: Allah is Shahid between you and I; this Qur'an has been revealed to me that I may warn therewith - you and whomsoever it may reach.”
[al-An'am (6): 19]
“It is He Who sent His Messenger with the guidance and the Religion of Truth that He make it prevail over all religions. And sufficient is Allah as a Shahid.”
[al-Fath (48): 28]
From “Explanation to the Beautiful and Perfect Names of Allah” — Shaykh as-Sa’di (d.1376H)