With Glossaries you can define any number of words and terms and give each a standard, centrally controlled, definition. The text you want to provide more information on is highlighted on your page. When the browser hovers the cursor over the highlighted text they’re presented with more information. For example:
Once the appropriate privileges have been granted, configure the Glossary Module to determine whether or not items are Glossarised automatically. Next, add glossary items. Finally, Glossarise pages to create references to Glossary Items.