A video game
Halo is a science fiction, first-person shooter video game published by Microsoft Game Studios. The trilogy centers around Master Chief, a enhanced super-soldier and his companion Cortana. Together they help the future in battling the Covenant, a theocratic collective of alien races. Halo refers to the Halo megastructures.
The popularity of the game has spawned the Halo into other realms such as books, graphic novels, soundtracks and other licensed products as well as a real-time strategy game, Halo Wars.