A piece of optical media, a round disc (e.g. a Compact Disc, Digital Versatile Disc, or Laserdisc), used as powerless persistent memory storage. Though special chemicals and techniques can be used to make rewritable optical media, most are read-only.
Optical discs are read in a hardware drive with three key mechanisms: a rotating spindle the disc is mounted on, a laser that is projected onto the disc and reflected back (into a sensor in the same assembly), and an arm that the laser itself is on, which can move back and forth. Thus, by spinning the disc and adjusting the arm, the drive can use the laser to access any part of the disc. Data is interpreted as bits depending on how the laser reflects, which in turn depends on indentations in the media.
Developing generations of optical media use finer and finer indentations and more accurate lasers to make the information itself physically denser.
A relatively new technology is the dual-layer HD DVD. The different layers are accessed by the laser being focused for the different layers and increasing or decreasing intensity of the laser to read or bypass the translucent first layer.
The latest generation is Blu-Ray.