A singleton as defined by Nick Bostrom is "a world order in which there is a single decision-making agency at the highest level." Any stable social order may be a singleton provided it is effectively unopposed within its domain, though the term is often used to refer to a government run by a superintelligence.
In the 2000 video game Deus Ex, the first player character is given the opportunity to merge with a globally-distributed artificial intelligence with the goal of governing more effectively than both existing states and competing conspiracies. Its 2003 sequel Deus Ex: Invisible War builds on this, with in-game references and conversation  contrasting it with historical commentary on democracy, and implies evolution to a singleton (modeled as an instantaneous democracy or noocracy) in one ending.
The worlds of Charles Stross novels Singularity Sky and Iron Sunrise are ultimately ruled by The Eschaton, an "ensemble intelligence" capable of sending information to its own past using a closed timelike curve, which it is depicted using for both recursive self-improvement and preventing the creation of competing superintelligences.