Singleton refers to a world order in which there is a single decision-making agency at the highest level. Among its powers would be the ability to prevent any threats (internal or external) to its own existence and supremacy, and the ability to exert effective control over major features of its domain.
A singleton is a more general concept than AGI-initiated intelligence explosion. For example, a democratic world republic could be a kind of singleton, as could a world dictatorship.