"Xenomorph" comes from two greek words - "xenos", denoting strange or foreign-ness, and "morphe", meaning shape. The reason these words were applied to the Aliens in the movies of the same names are because the colonial marines didn't know what they looked like (or rather, of their wide range of appearances.)
GORMAN
What's the question?
HUDSON
Is this going to be a stand-up
fight, Sir, or another bug-hunt?
GORMAN
All we know is that there's
still no contact with the colony
and that a xenomorph may be
involved.
WIERZBOWSKI
A what?
HICKS
(to Wierzbowski;
low)
It's a bug-hunt.