Double density
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Double density usually refers to a physical format in a magnetic storage system that uses twice as many bits per length unit as the basic format.
In 3½ inch floppy disks using IBM PC compatible MFM format, double density marks a disk format that uses 9 sectors of 512 bytes per track, for a total capacity of 720kB in a two-sided 80 track floppy disk. 3½ inch floppy disks most commonly used in the latter 1990s and the 2000s were high density, which increased the capacity to approximately 1440kB.
The term double density can also be applied to magnetic tapes.