Apple high-density 3 1/2" disk
The Apple high-density 3 1/2" disk was used on the Macintosh (with the MFS or HFS file system) and the Apple II line (with the ProDOS file system, requiring ProDOS 8), as a higher-capacity format then the previous Apple double-density 3 1/2" disk. It was a disk format with 80 tracks per side and 18 sectors per track, with each sector storing 512 bytes. Disks were double sided, for a total capacity of 1440 KB. Data was encoded using MFM encoding.
3 1/2" disks are actually 90 mm wide, but are almost universally referred to as "3 1/2 inch" disks even in countries that use the metric system.