Xenos is an Imperial term synonymous with “alien” (from the human perspective), and refers to all non-human and extraterrestrial sentient species. “Xeno” is properly only used as a prefix. A xeno-artefact or xeno-technology for instance respectively refers to an alien artifact or alien technology.
The major alien races are the Orks, Eldar, Dark Eldar, Tyranids, Necrons and the Tau. All these races pose a considerable threat to the survival and continued dominance of mankind. There are countless other races but all are trivial by comparison.
The Imperium considers humanity the rightful rulers of the whole galaxy. Part of Imperial Creed is that all aliens judged to pose a threat to mankind’s survival must be contained or destroyed. Aliens – along with mutants, heretics and deviants – are all considered enemies of mankind – for they threaten the very survival of the human race.
Many new alien races and civilizations are discovered by Imperial Explorators every year. For the most part these aliens are ignored, contained, or if they present a threat to mankind or the Imperium, eradicated. Throughout its history the Imperium has fought many long wars with dangerous alien species. In most cases the Imperium has eventually won, but in some, the threat has only been managed to be contained.
Although friendly dealings with aliens is almost unthinkable, Rogue Traders are allowed to barter and trade with aliens.