Although the word Aten is primarily known as the name of a god, originally it only designated the sun disk as a celestial body; as such the word was already used in the Middle Kingdom. But soon afterwards the sun disk was personified and in the early New Kingdom an extra hieroglyph was added to its writing to indicate divinity.
It was king Amenhotep IV (also known as Akhenaten) who went one step further by declaring the Aten to be the only god, at the same time denying the existence of other gods.
The full name of the god (which was adapted a couple of times during the reign of Amenhotep IV) was much longer than just the word Aten. Originally (at the accession of the king) it was "May Re-Harakhte live, who rejoices in the horizon in his name of Shu, who is the Aten". Later the reference to other gods was deleted from the name.
Early depictions of the god show him as a man with the head of a falcon, wearing a sun disk with uraeus on its head. But very soon afterwards only the sun disk was depicted, having numerous arms (ending in hands) instead of sunbeams. Often these hands hold the sign of life to the nose of the king and queen.