A .mobi file is a format used for digital eBooks, primarily associated with Amazon Kindle devices. It's designed to contain text, images, and simple formatting optimized for reading on small screens. Unlike open formats like EPUB, .mobi (based on the MOBIpocket standard) traditionally used DRM protection and had more limitations in supporting complex layouts, advanced styling, or interactivity, focusing instead on ensuring readability across older e-ink devices.

Its main use was for reading eBooks purchased from or distributed through Amazon on Kindle e-readers and Kindle apps. Users could also convert personal documents (like Word or PDF files) to .mobi format via Amazon's email service ("Send to Kindle") for reading on their Kindle devices before newer standards became dominant.

As a proprietary format largely succeeded by Amazon's own KF8/AZW3 format and the industry-standard EPUB, .mobi files offer limited compatibility outside the Kindle ecosystem. Their main advantage was early Kindle device support, but limitations in rendering modern layouts and reduced industry adoption outside Amazon have diminished their relevance. While Amazon still accepts .mobi for conversion, EPUB is now preferred for wider compatibility and richer formatting capabilities.