The Immortal appear to be a sub-species of Human that possessed immortality, or at least were very difficult, if not impossible, to kill. Immortals are very similar to Humans, yet cease ageing at a point, and can heal from all injuries almost instantaneously.

Evidently, one cannot know if they are an Immortal until their death. (TNG novel: Greater Than the Sum)

Gary Seven was aware of the existence of Immortals prior to 1984. (TOS novel: The Rise and Fall of Khan Noonien Singh, Volume 1)

Known ImmortalsEdit