Google Dictionary has an undocumented API that's used in Google Docs. You can obtain a JSON output using a URL like:
(replace test with your favorite keyword).
A developer built a Chrome extension that used Google Dictionary API to display the definitions of a word without opening a new page.
A few days after releasing the extension, Google asked the developer to stop using the API.
"Until recently, this extension was known as Google Dictionary Lookup and used as its data source the Google Dictionary. Unfortunately, due to Google's contracts with their data providers, they are not legally allowed to expose the Dictionary API to third parties, and as such, I was forced to switch providers. (...) In the spirit of FOSS, I'm leaving the source code of the previous version available at my site until Dec 25, but you are solely responsible for its use, and I urge you not to abuse the Google Dictionary API," explains Max.
If you'd like to see an official API for Google Dictionary, star this issue.
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