I don't think there is a visually appealing way to show info like this if you keep the 'hidden term' on screen all the time. Having it sit under the line will not help the visual flow of the content on the page. This would be better to display when the user hovers their cursor over the term. I would make use of the hyperlink functionality to display a popup.

To have a look at what this looks like type your line of text and select the text eg 'World Wide Web' and press Ctrl-K to open the Hyperlink dialog. In that dialog, click the Screentip button and add the acronym that you want to see when you hover over the text (www). You won't be able to actually insert this hyperlink unless you add something to the address field but it doesn't have to be a valid url (you can just put in a couple of random characters if the link doesn't matter).

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Andrew Lockton
Chrysalis Design, Melbourne Australia