magnetic disk

A memory device, such as a floppy disk, a hard disk, or a removable cartridge, that is covered with a magnetic coating on which digital information is stored in the form of microscopically small, magnetized needles.

A storage device, consisting of magnetically coated disks, on the surfaces of which information is stored in the form of magnetic spots arranged in a manner to represent binary data.

The noun has one meaning:

Meaning #1: (computer science) a memory device consisting of a flat disk covered with a magnetic coating on which information is stored
  Synonyms: magnetic disk, disk, disc