A ring around a character's head, normally seen decorating holy beings. More modern representations often take the form of a frisbee-sized ring hovering above the head, older religious iconography often depicts it face-on to the viewer.
The name is also used for an optical phenomenon which circles the sun with a ring of light, and there's another optical phenomenon called a glory where a rainbow ring surrounds the head of the viewer's cast shadow. For tagging art, anything bright or rainbowish haloing the head makes this tag applicable. Artistically it's a good way of picking out the main character or flagging a character's head for attention, as you'd expect.
The Halo Franchise is an Xbox and PC game featuring a battle between the Alien group, the Covenant, and the Human Colonies.