Any of a variety of flat, circular, information storage media, ranging from old vinyl records to the common 3.5-inch floppy disks. In computer parlance, disk typically refers to a flat, round surface covered with a magnetic material (such as a magnetic oxide) which can store digital (or other) information. This magnetic coating can be manipulated by the writing device (or write head) to carry magnetic fields, the varying orientations of which can be interpreted digitally by the reading device (or read head) and translated into machine- or human-readable code. This is how a vast majority of computer data is stored.
Although often used interchangeably with the term disc, this latter term is reserved for media that store data optically, such as compact discs.