An Eternal is a person born with natural immortality, caused by a rare genetic disorder of mysterious origins. Due to the spread of these genes, the number of potential Eternals has only grown throughout human history, along with overall human population numbers.
Besides longevity, the main difference between an Eternal and an Ephemeral (mortal person) are the extreme regenerative capabilities of the former. An Eternal dying for the first time results in an Eternal's first regeneration. This has the potentionally problematic side effect of permanently locking the Eternal at the age he or she had when he or she died for the first time.
Killing an Eternal completely means killing such a person permanently. However, to date, such a thing has been achieved only rarely. This is because the regenerative abilities of an average Eternal are immense, and are hard to easily negate or disrupt for long enough to make the Eternal in question die.