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 3.5" Floppy disks 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.
Curently (2003) used 3.5" floppy disks are high density, which doubles the capacity and bit density of double density.
The term double density can also be applied to magnetic tapes.