A flat rotating disc covered on one or both sides with some magnetisable material. The two main types are the hard disk and the floppy disk.

Small areas or "zones" on a magnetic disk are magnetised. The magnetisation is aligned in one of two opposing orientations with respect to the recording head. The head writes data by altering the alignment and reads data by detecting current pulses induced in a coil as zones with different magnetic alignment pass underneath it.

Data is stored on either or both surfaces of discs in concentric rings called "tracks". Each track is divided into a whole number of "sectors". Where multiple (rigid) discs are mounted on the same axle the set of tracks at the same radius on all their surfaces is known as a "cylinder".

Data is read and written by a disk drive which rotates the discs and positions the read/write "heads" over the desired track(s). The latter radial movement is known as "seeking". There is usually one head for each surface that stores data though, for increased performance (from reduced rotational latency), it is possible, though expensive, to position multiple heads at equally spaced angles around the discs.

Therefore there are two states that can be detected for each zone - a change in alignment, or no change.

Ideally a data bit of one or zero can be recorded in each zone of magnetisation, however, if a zero represents an absence of magnetic change, the detection of several consecutive zeros would imply accurately measuring the length of time of the absence, i.e., measuring the time between two magnetic changes. However, the variability of motor speed limits the number of consecutive zeros which can be read reliably.

The best recording methods accurately follow the characteristics of the magnetic and rotational aspects in recording the disk, to be as dense as possible in recording bits.

Compare magnetic drum, compact disc, optical disk, magneto-optical disk.