An Eternal is a sub-species of any given race engineered by the Celestials to maximize their genetic potential.
Eternal variants exist for most major races.
Eternals typically have vastly increased physical and mental capabilities over their less-evolved brethren. They're used as a control group for Celestial bio-engineering to ensure that selected races can eventually achieve genetic apotheosis.
In the Sol star system, Eternals predate human civilization by some 700,000 years. The most notable of Eternals is Thanos the Conqueror. Homo Immortalis occupies Saturn's moon Titan and have maintained interstellar contact with many other races for their tenure there.