A glowing light that circles something, like the moon or a person's head is a halo. Painters of religious art often put a halo around the heads of angels and saints.
A halo is a symbol of holiness, represented by a circle or arc of light around the head of a saint or holy person. Any circle of light can be described as a halo, even in a non-religious context; sometimes you'll see a glowing halo of light around the moon, for example. The word halo also means "glory or majesty," a symbolic halo rather than a physical one. The Greek halos means "ring of light around the sun or moon."