A Tooltip from hovering over Elyssia Winterdusk.

When you hover your mouse over certain items (other characters, NPCs, mobs, items, buttons, etc.) a small window will pop up displaying a description of the item. This also works in many other computer programs. For example, hold your mouse over the "W" logo image (This is a tooltip ← this one should show "This is a tooltip") in the upper-left, and a small box saying "Wowpedia" will appear.

This term comes from general computer UI tool tips from buttons or icons.

In Wowpedia

See Help:Tooltips for information about how to make tooltips work on special links in Wowpedia.