Tranquility is a CCP server cluster that serves the global EVE playerbase (with the exception of China). Tranquility runs the main EVE instance, and as such is continuously updated.
Tranquility is located in London, and is therefore near the international prime meridian that helps to define Coordinated Universal Time ("UTC") and so, in turn, Eve Server Time.
Although Tranquility sustains one interconnected, persistent world and is experienced by players as a single unified "server", it is in practice separated into different nodes, which are individual units of hardware hosting particular parts of New Eden.
Like all other CCP server clusters, Tranquility experiences server downtime once each day.