YouTube English Definitions processes YouTube subtitle text locally in the user's browser.

When the user hovers over or clicks a subtitle word, that word is sent to the Datamuse API solely to retrieve an English definition. Video URLs, browsing history, YouTube account information, cookies, names, email addresses, and other personal information are not transmitted.

Extension preferences are stored locally using Firefox storage and are not sent to the developer. The extension does not use analytics, advertising, tracking, or user accounts.

Datamuse temporarily logs API queries and deletes these logs at the end of the day. The developer does not receive or retain these queries.

Datamuse API information:
https://www.datamuse.com/api/