I think it's a great, simple application. I think my vocabulary is a bit extensive for it (did a dozen or so, got them all), but it would still be useful.

My comments: a) The animation is distracting, b) I didn't know at first that I was supposed to click a definition (you should add a "click the right one" instruction), c) I wish there were keyboard shortcuts to pick an answer, d) When you mouseover the answers the background changes, yet you can't click the background [1].

As for monetization, you may have a market with students studying for spelling bees or standardized tests or whatever. Some parents would spend ridiculous amounts of money if they think it will help their children.

[1] EDIT: Ignore this one. I don't know why I said that you can't click the background.