Aten was the focus of Akhenaten's religion, but viewing Aten as Akhenaten's god is a simplification.
Aten is the name given to represent the solar disc.
The term Aten was used to designate a disc, and since the sun was a disc, gradually became associated with solar deities.
Aten expresses indirectly the life-giving force of light.
The full title of Akhenaten's god was The Rahorus who rejoices in the horizon, in his/her Name of the Light
which is seen in the sun disc. (This is the title of the god as it appears on the numerous stelae which were
placed to mark the boundaries of Akhenaten's new capital at Amarna, or "Akhetaten.")
This lengthy name was often shortened to Ra-Horus-Aten or just Aten in many texts, but the god of Akhen-
aten raised to supremacy is considered a synthesis of very ancient gods viewed in a new and different way.
Both Ra and Horus characteristics are part of the god, but the god is also considered to be both masculine
and feminine simultaneously.
All creation was thought to emanate from the god and to exist within the god.
In particular, the god was not depicted in anthropomorphic (human) form, but as rays of light extending
from the sun's disk. Furthermore, the god's name came to be written within a cartouche, along with the
titles normally given to a Pharaoh, another break with ancient tradition.
The Aten first appears in texts dating to the 12th dynasty, in The Story of Sinuhe.
Ra-Horus, more usually referred to as Ra-Herakhty (Ra, who is Horus of the two horizons), is a synthesis of two other gods, both of which are attested from very early on. During the Amarna period, this synthesis was seen
as the invisible source of energy of the sun god, of which the visible manifestation was the Aten, the solar disk.
Thus Ra-Horus-Aten was a development of old ideas which came gradually.
The real change is the apparent abandonment of all other gods following the advent of Akhenaten, i.e., the introduction, apparently by Akhenaten, of monotheism.
This is readily apparent in the Great Hymn to the Aten.