1.A mythical ring of gold or light that floats a few inches over an Angels head. Can be seen in the cartoon DBZ.
2.A large Metal ring in space, spanning 10,000KM in diameter and 22.3 KM thick. Was constructed by Forerunners to house the parasitic plague called the Flood. SPins to generate gravity.
1.Hey, is that a Halo on your head?
2."Were all that's left.Halo,it's finished" -Cortana