A singleton refers to a world order in which there is a single decision-making agency at the highest level, capable of exerting effective control over its domain and preventing internal or external threats to its supremacy. An artificial general intelligence having undergone an intelligence explosion could form a singleton, as could a world government armed with mind control and social surveillance technologies. A singleton doesn't have to support a civilization, and in fact may obliterate it upon coming to power.
A singleton need not directly micromanage everything in its domain; it could allow diverse forms of organization within itself, albeit guaranteed to function within strict parameters. Notably, a suitable singleton could solve world coordination problems that would not otherwise be solvable, opening up otherwise unavailable developmental trajectories for civilization. For example, such singleton could hold Darwinian evolutionary pressures in check.