There is an answer. It has to do with creating two buttons and then using a rollover on one button (the word) to show the other (the definition).
It's a little complicated to write out all the steps here, but if you search for "remote rollover InDesign" you will find a lot of resources. But don't get into the animation ones. They won't work with PDFs.
One thing you should know, when "scrolling over" to me means a rollover. That's when the cursor hovers over a position on the page. But PDFs viewed on tablets can't have rollovers. So if you want it to work on tablets you have to tap. And it's a little more difficult to make the definition disappear.
I will definitely look into this, Sandee. Thanks for helping me out.