Halo is a science fiction video game franchise created by Bungie and owned and published by Microsoft Game Studios. The main trilogy of games center on the experiences of the Master Chief, a cybernetically-enhanced human super-soldier, and his artificial intelligence (AI) companion, Cortana. The Master Chief aids future humanity in battling the Covenant, a theocratic alliance of alien races. In this science fiction setting, the term "Halo" refers to Halo megastructures: large, habitable ringed structures, similar to Larry Niven's ringworlds.

The trilogy of video games have been praised as being among the best first-person shooters on a video game console, and are considered the Microsoft Xbox's "killer app".  This has led to the term "Halo killer" being used to describe console games that aspire, or are considered, to be better than Halo.  Fuelled by the success of Halo: Combat Evolved, and immense marketing campaigns from publisher Microsoft, both sequels went on to break various sales records. Halo 3 sold more than US$170 million worth of copies in the first twenty-four hours of release, breaking the record set by Halo 2 three years prior.  The trilogy of games have sold over 24.8 million copies worldwide.

Strong sales of the games has led to the franchise's expansion to other mediums; there are five bestselling novels, two graphic novels, and other licensed products. Beyond the original trilogy, other "Halo" games have branched off into other video game genres, including Halo Wars, a real-time strategy game produced as developer Ensemble Studios' last project. Video games in development include Halo 3: ODST and Halo: Reach. The series' award-winning music was composed by Bungie's own in-house musician Martin O'Donnell, and soundtracks have been released for the entire trilogy. The cultural impact of the Halo series has been compared by writer Brian Bendis to that of Star Wars. The collective group of fans of the series is referred to as the "Halo Nation". (via Wikipedia)