An / Anu, the Sky-Father
The sky itself, eldest of the gods — supreme in honor, yet too lofty to act.
Also: An · Anu · sky god · Anum · father of the gods · Uruk
An — Anu in Akkadian — is the sky itself personified, the oldest and most exalted of the gods and the formal head of the pantheon. From him flows authority: kingship and the highest honors are said to be granted 'by An,' and to be raised up by An is the ultimate legitimation. Yet he is a distant, dignified figure who seldom acts directly in the stories; the practical governance of the cosmos falls to Enlil, and later Marduk. An embodies sovereignty in its purest, most remote form — the source of legitimacy that stands above the fray.
In short
An (Sumerian) / Anu (Akkadian), the remote sky-god and nominal head of the pantheon, source of authority and legitimacy though rarely an active agent in the myths.
Where it’s found
Pervasive across the Sumerian corpus; e.g. the granting of authority in 'Enki and the world order' (ETCSL 1.1.3, in corpus); Enūma Eliš for Anu's place (akkadian-pending)
How the idea developed across Sumer, Babylon & Assyria
An/Anu heads the formal hierarchy throughout, but real executive power migrates from him to Enlil in Sumerian theology and to Marduk (Babylon) / Aššur (Assyria) later. His city is Uruk, shared with Inana.