Atemporals are immortal human souls. There are two mechanisms of immortality discussed in the Mitchell universe, primarily discussed in The Bone Clocks.
The soul of the first variety of Atemporal persists after the death of its human "host" and returns to a new human body (this is in contrast to normal souls, which are said to travel into a Last Sea beyond the Dusk rather than return to life). There are two subcategories to this variety: Returnees and Sojourners. The souls of Returnees return to a new human body 49 days after the death of their host body. The souls of Sojourners leave their human body upon death and move directly to the body of another. Both appear to take the bodies of children who are on the verge of death whose souls have freshly vacated their bodies. Neither Returnees nor Sojourners appear to have a choice about whether their souls remain.
The second variety of Atemporal are "self-elected" and gain immortality by consuming the souls of others, preventing their bodies from aging. They are described at various points in the novels The Bone Clocks and Slade House as Soul Carnivores, or simply Carnivores.
Both varieties of Atemporals practice psychosoterica, a broad collection of supernatural, psychic abilities. The abilities are separated by the so-called Schism into two warring factions: practitioners of the Deep Stream (Le Courant Profond), associated with the more benign Atemporals (most notably Horology), and the Shaded Way (La Voie Ombragée), associated with the Carnivores.