Image from Anders Sandberg
Generic term for alien (non-terragen) lifeform. May be animal-like, plant-like, protistan, exotic chemistry, or any other possibility. Does not have to be sophont. In fact, just as on Earth, very few alien species ever evolve any measure of cognitive intelligence.
While some display broadly parallel evolutionary traits with terrestrial creatures, others are completely unlike any Terragen life. May be found in any environment that can support life, include extremophile conditions deadly to most terragen organisms. The number of different xenobiont species discovered and recorded so far numbers in the trillions, but this is still but a tiny percentage of total species of life-forms to be found in our galaxy alone. Also, even planets that have been mapped and surveyed may contain many undiscovered or unrecognized species (especially rare or microscopic forms).
Many natural garden worlds with their complements of alien life forms have been claimed by corporations or caretaker gods, the former marketing select species as pets or novelty value, the latter jealously guarding the indigenous biota from appropriation by other sentients.
Source : http://www.orionsarm.com/eg-article/46f1dd478722c