An optical disk is a recording medium on which data is encoded as microscopic pits in a flat surface. The data is stored on the medium with a laser or stamping machine and can be accessed when the data path is illuminated with a laser.

An optical disk (for example, a CD or DVD) is a flat disk which can contain data encoded as microscopic pits in a special encoding material on one of its flat surfaces. These pits distort reflected laser light to represent the data when the disk is being read. The encoding material resides on top of a thicker substrate that constitutes the bulk of the disk. The encoding pattern follows a continuous spiral path covering the entire disk surface and extending from the innermost track to the outermost track. The data is stored on the disk with a laser or stamping machine and can be accessed when the data path is illuminated with a laser diode in an optical disk drive.

Most optical disks do not have an integrated protective casing and are therefore susceptible to data deterioration due to scratches, fingerprints and other environmental problems.

An optical disk is designed to support one of three recording types:

• read-only (e.g. CD and CD-ROM)
• recordable or write-once (e.g. CD-R)
• re-recordable or rewritable (e.g. CD-RW).