Add to Glossary
Go to the "Glossary acronyms" in the left navigation, then edit page using 'edit source'.
Once there you can add a new line of content in the format:
;ABBR:full text of abbreviation
so it looks like this
- Geographic Information Services
- Global Positioning System
After that, any time the abbreviation is used on another page, it will automatically be applied with the full description as a hover link.
This will take half an hour or so to actually show up as it seems to search the pages and remember things!