The film's title Alien obviously refers to the creature of this feature, the alien. But for a movie that is so important to film history, doesn't that title seem kind of… bland? It'd be like naming a romantic comedy Valentine's Day or a movie about an airplane Airplane!.
Consider some alternatives, like Invasion of the Body Snatchers. The name "body snatchers" straight up tells you that they snatch bodies and the inclusion of "invasion" clues us in that they will be showing up uninvited. Then there's Earth vs. the Flying Saucers, which tells us that we're about to see a rumble in the galactic jungle. You know right away that there will be scenes depicting tanks and fighter jets against metal pie pans in the sky.
In both cases, everything we need to know about movie is spelled in the title. Our viewing experience is really just to see how things play out.
So… why Alien?
It's precisely because the word is so simple. All we know about it from the title is that it is something foreign and different. It could be anything and do anything. Let your imagination run with the unknown and things get real creepy, real fast.
The title reflects the film's obsession with the fear of the unknown. The alien is so, er, alien that it can't even be named. We don't go in knowing that bodies will be snatched or saucers will fly. We only know that we're going to confront something profoundly, weirdly, and lethally inhuman.