Halo is a science fiction video game franchise created by Bungie and now managed by 343 Industries and owned by Microsoft Studios.
In "The Dumpling Paradox", the gang is playing Halo on Halo Night when Penny's friend Christy gets Howard's attention and he ditches them. Penny joins in and finds that she is a great Halo player. The next week, while Howard still has Christy's attention, the guys try to get Penny back to play Halo, but she is going dancing that night. The guys then go to get Howard at the Wolowitz House and hear Howard's mother arguing with the couple. Christy leaves when Mrs. W tells her that he's cut from her will.
In "The Cooper-Nowitzki Theorem", Leonard comes home for Halo Night and Sheldon's self-appointed assistant Ramona Nowitzki will not let him participate in any of his leisure activities including Halo.