Alien (異星人, Iseijin?) is a living thing who live at further universe, which is almost unknown or undiscovered by humans, especially scientists. Humans often having curiosity to know more aliens' life styles, but most of them believe that aliens are having advanced knowledge on technologies, for example creating UFO that can travel million miles, even light years.
In Urusei Yatsura, there are different types of races for this topic. Oni, Yuki on'na and Shichifukujin are the popular alien races who appear in this series. Although some aliens are having exact appearance like humans, they are also can be in the form of animals, or undefined shape shifting like creature.