In general use, an alien is any living being which is not native to the location where it is found. In science fiction terms, this usually means a being that comes from a planet different from the primary setting. If the story takes place in outer space and most of the characters are human, "alien" may just generically refer to all non-humans in the story.
Alien is also the title of a 1979 sci-fi/horror movie starring Sigourney Weaver that spawned several sequels. The monsters of this series are widely referred to as "aliens" in sci-fi discussions.
The monsters from the 1979 movie (described as "xenomorphs" in the first sequel) go through three stages in the films.
Their eggs are usually the first form encountered. Each egg is an ovoid casing roughly 60cm long that contains a larval alien. Eggs can apparently remain dormant for extended periods of time, but they are sensitive to some external stimulus -- it is not known if it is heat, motion, sound, or something else -- that causes them to open.
When an egg opens, a larval alien form emerges. The larval or "face-hugger" form is approximately the size of a small dog or a cat. It has a flat body with eight finger-like legs, a muscular tail, and a tube-like organ on its ventral side. The larvae seek out animals to attack, leaping onto their victims' heads and gripping with their legs and tail. The larva then forces its ventral tube into the victim's mouth, rendering the victim unconscious. The larva remains attached for several hours, feeding air to its victim during this time. While attached, the larva implants an embryonic form of the next alien life-stage in the victim. After implanting the embryo, the larva drops off of its victim and dies.
The embryo implanted in the victim grows over the course of several hours into a miniature version of the adult form. Having reached the size of a small cat, it tears its way out of the victim's torso, causing its "host" a painful and gory death. After emerging, the alien typically runs away from any potential threats and finds a place to hide while it grows to full size. It presumably hunts small prey and scavenges any suitable organic material in the vicinity to fuel this growth.
The alien reaches its full adult size several hours after emerging from the host. The adult is built vaguely like a terrestrial predator, with four legs, a tail, and a large head with powerful jaws. Like bears, they typically move on all fours, but they often rise onto their hind legs to fight. The forelegs of the adult end in powerful hands, and the feet of the hind legs are also prehensile, making aliens strong climbers. The adult's tail ends in a large, blade-like stinger. The elongated head has few recognizable features; there are no visible eyes, ears, or nostrils. The jaws, however, are quite prominent and filled with spike-like teeth. The animal's head also contains a second, smaller set of jaws that can be quickly and forcefully extended 30 centimeters or more out of the creature's mouth. The adult form is extremely strong and a powerful leaper.
In all forms, aliens have extremely caustic body fluids that can dissolve metal and human tissue easily, making it dangerous to attack them from close range with anything that might break their skin.
Aliens are eusocial animals that quickly form into colonies. An individual specimen will attempt to capture another victim to be infected by another larva, which may develop into a queen (the exact conditions that cause a queen to develop in a host are unclear).
Queens grow much larger than typical adults and develop an enormous egg-laying abdomen. The abdomen is not essential for the queen to survive, however, and she can tear herself free from it if threatened. Other adult aliens will seek food and hosts for the colony while the queen -- protected by a few adults -- produces eggs to expand the alien population.
Aliens wouldn't pose any serious threat to most sci-fi civilizations. For the most part, they only pose a threat to unprepared civilians: they're no-match for a coherent military unit with automatic firearms unless they have a huge numerical advantage (which they had in their second movie, Aliens, and the Colonial Marines they were fighting -- despite some tactical blunders -- still killed many of them).
Aliens in Debates
Discussing how various sci-fi cultures and military forces would fare in the face of an alien infestation is a popular topic of sci-fi debates.
One common debate match, Aliens versus Predators, eventually became a movie.
- Alien at IMDB.