HALO - (nimbus, type of mandorla) A circle of light pictured around the head of a saint to show holiness. The halo represents the light of divine grace suffusing the soul, which is perfectly united and in harmony with the physical body.
In the theology of the Eastern Orthodox Church, an icon is a "window into heaven" through which Christ and the Saints in heaven can be seen and communicated with. The gold background of the icon indicates that what is depicted is in heaven. The halo is a symbol of the Uncreated Light (Greek: Ἄκτιστον Φῶς) or grace of God shining forth through the icon.