Xenomorph XX121, commonly referred to as simply the Xenomorph (ZEE-no-morph), and known colloquially as the Alien, is a highly aggressive endoparasitoid extraterrestrial species. The Xenomorphs are vicious predatory creatures with no higher goals than the propagation of their species and the destruction of any life that could pose a threat to them. Like wasps or bees, Xenomorphs are eusocial, with a fertile Queen breeding a host of subordinate castes. The creatures are known for their potent acidic blood and their pharyngeal jaws, although their biological life cycle, in which their offspring are implanted inside living hosts before erupting violently from their chests, is in many ways their signature aspect.

Aside from their repeated conflicts with humans, Xenomorphs have notably been associated with the Yautja, who know them as kiande amedha ("hard meat").The Yautja revere the creatures as incredibly dangerous and thus highly worthy prey, notably employing them in initiation Hunts that serve to test young members of their society and mark their progression into adulthood.