I decided it was about time for one, though this one doesn't have any cross links or anything like that yet, here it is:
http://ansible.github.com/glossary.html
edits to the "rst/" directory of the docs project are welcome to add terms, fix formatting, cleanup errors, etc (as with the rest of the docs project).
I thought it was saying something not-good that I think we needed one, in hopes that it was all super super obvious, but on the other hand, some place to lightly see everything mentioned all in one place seemed to also be a good thing.
Edits go to:
https://github.com/ansible/ansible.github.com/tree/master/rst
--Michael