The One who witnesses the saying and deeds of His creatures.
Allah سُبْحَٰنَهُۥ وَتَعَٰلَىٰ is Al Muhaymin (in Arabic: ٱلْمُهَيْمِنُ) meaning the one who ensures well being and protection over His creation. He is also the ever watchful. Nothing that can be hidden from Him, He is the knower of all the secrets we conceal.
From the root h-y-m-n which has the following classical Arabic connotations: to watch over, oversee, protect, guard to be a witness to offer security and peace to determine what is true, to extend a wing. Similar to the Arabic expression, 'haymana al-ta'ir alafirdkhih' meaning that the bird took its young ones under the protection of its wings.
The name al-Muhaymin refers to His supreme nature as being the guardian or the absolute authority. He is the superseder, nothing will overtake or usurp Him. He is the ever-watcher, not just limited to the superficial, He can see all which is concealed in the hearts. Nothing can be kept a secret from Him. "He knows what is in the heavens and earth; He knows what you conceal and what you reveal; God knows very well the secrets of every heart." (64:4)
You can find variations of the word muhaymin, like muhaiminan in Surah Maidah verse 48 where Allah سُبْحَٰنَهُۥ وَتَعَٰلَىٰ declares the Qur'an as being the book that will be guarded / dominate over the other books:
Muhsin Khan Translation:
"And We have sent down to you (O Muhammad SAW) the Book (this Quran) in truth, confirming the Scripture that came before it and Mohayminan (trustworthy in highness and a witness) over it (old Scriptures)."
But as a name and attribute of Allah سُبْحَٰنَهُۥ وَتَعَٰلَىٰ, al-Muhaymin is only mentioned once in the Qur'an.
Huwal-laahul-lazee laaa Ilaaha illaa Huwal-Malikul Quddoosus-Salaamul Muminul Muhaiminul-‘aAzeezul Jabbaarul-Mutakabbir; Subhaanal laahi ‘Ammaa yushrikoon
"He is Allah, other than whom there is no deity, the Sovereign, the Pure, the Perfection, the Bestower of Faith, the Overseer, the Exalted in Might, the Compeller, the Superior. Exalted is Allah above whatever they associate with Him." (59:23)
The word هيمنة (transliteration: haymana) in modern Arabic means dominance. One country can seize another, being in control or showing dominance over that state. But this is often transitory, exchanging hands from one "owner" to another. Only Allah سُبْحَٰنَهُۥ وَتَعَٰلَىٰ is al-Muhaymin, the one who has complete dominance, a quality of being in control, supreme that lasts forever. It is not limited by any means.
But this is one form of dominance. Muhaymin also refers to an authoritative figure who reigns supreme, one that you can trust. This is analogous to a parent who out of love for their child will set rules and boundaries to protect and care for the child. It's often done in the best interest despite the kid thinking otherwise.
How can we benefit from knowing al-Muhaymin? Just as Allah سُبْحَٰنَهُۥ وَتَعَٰلَىٰ is the overseer of all his creation, we need to be watchful over ourselves. Many people do not know themselves - they think they are their thoughts. We can let horrible or negative thoughts come to mind, but we don't necessarily have to identify with them. We can let it come and pass, just like how a river flows. Identification, thinking you are your thoughts, is like a beaver building a dam in the river. This only ends up damaging the surrounding environment. Similarly, identification can be destructive, leading to either a depressive state or causing one to become arrogant or narcissistic. Instead, it is better to try and be indifferent. Take time to observe and watch your thoughts come and go, and ask why they may have arisen? Analyze their depths, and you may uncover secrets about yourself hiding in your psyche. Imam Ghazali recognizes this watching as a crucial step in reforming oneself which then can transform into becoming a guardian of his or her heart. Only selectively choosing thoughts that are constructive and letting everything else go.