From their website:
API Development Center - Welcome
Let's Make Money Together
If your app is commercial use (either monetized via ads or a paid fee) great! Please contact us directly at
[email protected] so we can approve use and develop business terms on a case-by-case basis.
We also offer a version of our API with ad tags if you would like to monetize your app through advertising served by Dictionary.com. Email us at
[email protected] if you are interested.