2. People in a country illegally.
3. Something that is foriegn to another something...or something what ever.
4. Something from another planet.
Butthead: Uhhhhhh, I believe in illegal aliens.
Beavis: Yeah me too. I think they are causing a rise in population and prices, economy.. or something.
Butthead: Quit trying to act smart. Dumbass.
Basically, if you have an alien Xenomorph on your planet, your species is doomed.