For over a decade, the . mobi format was synonymous with Amazon Kindle. If you bought an independent eBook, downloaded a classic from Project Gutenberg, or received an Advance Reader Copy (ARC), you loaded it onto your Kindle as a MOBI file.
Source: [Dev.to](https://dev.to/momahdideveloper/why-amazon-deprecated-mobi-for-kindle-and-how-in-browser-epub-conversion-works-30m0)