Xenobiology is the study of alien lifeforms and is a subdiscipline withing the Xenological sciences. It is often referred to as Astrobiology, a word coined in 1954 by science fiction writer Robert Heinlein in his work The Star Beast. Due to the fact no extraterrestrials have been found, xenobiology has been described as a "science in search of a subject."
Another XXX X_ology word
Xylology: The branch of dendrology treating of the structure of wood.
And that's it for today. Wish I had more for you guys, but X is a difficult letter. Those of you doing the A-Z challenge, did you find the post for X easy or difficult to write? Anyway, hang in there, you're in the final stretch! :D