type of computer memory in the form of a compact disc that is read by optical means. A CD-ROM drive uses a low-power laser beam to read digitized (binary) data that has been encoded in the form of tiny pits on an optical disk. The drive then feeds the data to a computer for processing.

The standard compact disc was introduced in 1982 for digital audio reproduction. But, because any type of information can be represented digitally, the standard CD was adapted by the computer industry, beginning in the mid-1980s, as a low-cost storage-and-distribution medium for large computer programs, graphics, and databases. With a storage capacity of 680 megabytes, the CD-ROM found rapid commercial acceptance as an alternative to so-called floppy disks (with a maximum capacity of 1.4 megabytes).

Unlike conventional magnetic storage technologies (e.g., tapes, floppy disks, and hard disks), CDs and CD-ROMs are not recordable—hence the tag “read only.” This limitation spurred the development of various recordable magnetic-optical hybrid storage devices; but they generally failed to penetrate beyond the publishing world, where large multimedia files are regularly exchanged, because of incompatibility with standard CD and CD-ROM players. In the early 1990s a new type of CD became available: CD-Recordable, or CD-R. These discs differ from regular CDs in having a light-sensitive organic dye layer which can be “burned” to produce a chemical “dark” spot, analogous to an ordinary CD's pits, that can be read by existing CD and CD-ROM players. Such CDs are also known as WORM discs, for “Write Once Read Many.” A rewritable version based on excitable crystals and known as CD-RW was introduced in the mid-1990s. Because both CD-R and CD-RW recorders originally required a computer to operate, they had limited acceptance outside of use as computer software and data backup devices.

To handle the proliferation of ever-larger multimedia files (audio, graphic, and video) in computer games, educational software, and electronic encyclopaedias—as well as high-definition movies for television entertainment systems—an expanded storage medium, digital videodisc (DVD (compact disc)), was introduced in 1995.