Alien

"Hello, we're a little lost..which way to the set of The Real Ghostbusters?"

An alien, in the broadest sense, is someone or something not native to where they currently are. In the common sense of most fiction, though, it means anything originating from an extraterrestrial source.

For example, a tiny, inferior, squishy creature may consider a giant, talking robot to be an alien, while at precisely the same time the robot could find the fleshling to be the alien.

Contrary to Hollywood, not all aliens have long heads and mouths on their tongues. Although some will scare the ever-loving crap out of you anyway.

From Teletraan I: The Transformers Wiki, a Wikia wiki.