noun: (video games) A game mode where the player character is invulnerable to damage, typically activated by entering a cheat code.
noun: (video games) A mode of play in (mostly) roguelike games, allowing the player to create objects on demand, to be resurrected in the case of death, etc.
noun: (by extension, slang) A temporary state of extreme skill (in a sport, video game, etc.).
