Aliens in Starfield are non-human life in all of its forms.
Overview[]
As no earthly creatures other than humans are present in Starfield, all nonhuman creatures are referred to as aliens. Most Earth species were either lost or did not survive the evacuation of Earth, and society has adapted to use alien species for foodstuffs and other roles. Alien fauna and flora aren't everywhere, but are very common on certain worlds like Jemison and Akila.
Starfield presents a game universe in which the only sapient (i.e. intelligent and advanced) life are humans. There is no record of humans encountering any pre-FTL or post-FTL intelligent life forms. The only debatable exception would be the Starborn, who aren't public knowledge. Starborn could also be viewed as 'ascended' humans or 'human adjacent' rather than something different entirely. While the game's plot states a predecessor of some sort in The Unity, the plot is very vague as to what that predecessor is and why it is no longer around. All the Unity has to show for its existence are the rare artifacts and temples. In any case, all the active societies in the Settled Systems are human, and there is no evidence to suggest otherwise.