Itn was Egyptian for the disc of the sun, and for most of Egyptian history it was a noun rather than a name. It appears in Middle Kingdom literature as the thing the sun-god crosses the sky in, and by the earlier Eighteenth Dynasty it had begun to be spoken of as a deity in its own right; Amenhotep III already gave it some prominence. What happened under his son was of a different order.
Amenhotep IV came to the throne about 1353 BCE, built an enormous open-court precinct for the Aten at Karnak in the shadow of the temple of Amun, and in his fifth year changed his own name to Akhenaten, "effective for the Aten", and moved the court to a new capital he founded on empty ground in Middle Egypt: Akhetaten, the horizon of the Aten, at the place now called Tell el-Amarna. He then went further than any Egyptian king had gone. The temples of the other gods were closed or defunded, their priesthoods dispersed, and the name of Amun, and in places the plural word for "gods" itself, was chiselled out of monuments the length of the country. The temples built for the Aten were roofless, open to the sky, because the god was not a statue in a dark shrine but the visible disc overhead.
The iconography is unlike anything else in Egyptian religion. The Aten has no body, no animal head, no consort and no myth. It is drawn as a disc with long straight rays, each ending in a small human hand, and many of those hands hold the ankh to the noses of Akhenaten, Nefertiti and their daughters. Those hands reach the royal family and nobody else, which is the theology stated in a picture: the god is universal, and access to him runs through the king.