Well, thanks for the responses. I guess the reason I asked is because, uhh.. Wouldn't it be possible to have the dictionary site optimized to have every word be a valid keyword search term? Don't hate me for asking this, but it's a really nice FYI.
there are many dictionaries that use adsense: dictionary.com thefreedictionary.com and answers.com i see two problems with the approach... the adsense revenue is not going to be very good... because what product or service is related to a definition page of some obscure word. your page with the definition of something like "lawnmower" will get some good ads, but most pages are not going to be like that. the second problem is getting traffic. there are tons of online dictionaries, so you are somehow going to have to figure out a way to distinguish yourself.
From my own experience, if you want to make an online dictionary for the sole purpose of earning from Adsense then don't go for it. Go for something which users can create the content for you, something that you don't have to keep updating its content yourself. But if you like the idea of making your own online dictionary and love to be involved in that kind of knowledge then go for it. Adsense will cover your hosting for it, plus a few bucks a days... unless it becomes very famous and many people start to rely on it, then, I think it will get you a decent revenue but that will take a heck of time... maybe several years. But if you have the good money to promote it well from the start, then it's not a bad idea. Good luck