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.
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.
In many Transformers continuities, in addition to humans and Cybertronians, there are both organic aliens and robotic aliens. Many of the organic ones look very human, and an awful lot of the robotic ones have the ability to transform like Cybertronians. The extent to which these similarities are coincidence varies from species to species, when it's explained at all.
| This page uses content from Teletraan I: The Transformers Wiki. The original article was at Alien.