What is the prefix for alien?

There isn't a single specific prefix for "alien". The meaning of "alien" itself already implies foreignness or difference.

However, depending on the context, you could use prefixes like:

* Extraterrestrial: This emphasizes the alien's origin from outside Earth.

* Non-human: This focuses on the alien's difference from humans.

* Xeno-: This Greek prefix means "foreign" or "strange" and is often used in scientific terms related to aliens.

Ultimately, the best prefix to use depends on the specific meaning you want to convey.