Christ Pantocrator
The face the prayer addresses.
Pigment suspended in hot wax — the technique of Roman funerary portraiture, still in use when this was painted and abandoned soon after. It survives because Sinai lay beyond the reach of the emperors who ordered icons destroyed in the eighth century, so the monastery kept a small group of panels that exist nowhere else.
The thing to look at is the face, and the way to look at it is to cover one half and then the other. They are not the same man. On one side the eye is wide and level, the brow calm, the mouth composed; on the other the eye is larger and darker, the brow heavier, the mouth pulled slightly down. Byzantine writers never mention the asymmetry, and the familiar modern reading — divinity on one side, humanity on the other — is a theology supplied by later viewers, not signed by the painter.
What is not in doubt is whose face this was taken to be. The Jesus Prayer names it: Lord Jesus Christ, Son of God, have mercy on me, a sinner — repeated until, in the hesychast phrase, it descends from the mind into the heart.