Aker (also known as Akeru) was an ancient Egyptian earth god and the deification of the horizon. He guarded the eastern and western borders of the netherworld and protected the sun god Ra when he first entered the netherworld at sunset and again when he returned to the world of the living at sunrise and bore the sun on his back through the underworld. He also welcomed the dead Pharaoh into the underworld. He defended the sun god from the serpent Apep (Apophis) and was considered to be able to neutralize a snake bite. In his role as a protective deity, twin lion statues representing Aker were placed at the doors of palaces and tombs to protect against evil spirits, a practice adopted by both the Greeks and Romans.

However, Aker in his plural form (Akeru) was more aggressive and dangerous. The Akeru are very ancient, possibly even older than Geb. The Pyramid Texts express the wish that each person will escape the clutches of the earth gods and specifically state that the Akeru will not attack the king (strongly implying that they could attack others).

The name Aker means "(one) who bends" but he was also known as "Ruti" ("two lions"). During the late period the two lions were given specific names - Tuau (today) and Sef (yesterday). Initially Aker was depicted as a narrow strip of land with a human or lion head at each end, but later the strip of land was replaced by the hieroglyphic sign meaning "horizon". The lions are often covered with leopard-like spots, possibly echoing the now extinct Barbary lion.