Apple double-density 3 1/2" disk
The Apple double-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). It was a disk format with 80 tracks per side and a variable 8 to 12 sectors per track, with each sector storing 512 bytes. When used in single-sided mode, its capacity was 400 kilobytes; this became 800 in double-sided mode. Data was encoded using a form of GCR encoding. The drive ran at constant linear velocity (CLV), meaning that the RPM rate varied depending on which track was being read.
The Apple high-density 3 1/2" disk is the high-density counterpart of this format, on higher-capacity physical media introduced later.
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.