CD-ROM CD read-only memory, a means of providing read-only access to a large amount of data for use on computer systems; the term applies to the medium in general and to a particular instance. Based on the 120 mm diameter audio CD, CD-ROMs are the predominant form of optical disk.

A CD-ROM drive must be used with the computer system to read the information from disk. Most drives can also play CD audio disks, but audio disk players cannot handle CD-ROMs. A standard CD-ROM, like the standard audio CD, is capable of holding about 640 Mbytes of data. The first CD-ROM drives moved the disk at the same speed as the audio product, both using CLV, and had a much lower rate – about 180 Kbytes per second – than hard magnetic disks (although higher than floppy disks). Double-speed and quad-speed drives are now the norm, with a proportionate increase in the data rate; most can also run at the standard speed.

The data on CD-ROMs is encoded in the form of a spiral of minute pits impressed into one surface of the disk at the time of manufacture, and cannot normally be rewritten (but see CD-R, CD-RW). The data may be in any form – text, sound, static or video images, or binary data, or a mixture (see multimedia); various CD-ROM format standards exist to handle these.

CD-ROM is widely used for the distribution of data, images, and software. Many commercial databases and indexes are available on CD-ROM, often as an alternative to an online service. CD-ROM is also being used increasingly, especially in personal computing, as an alternative to multiple floppy disks for the distribution of software, clip art, and fonts. For situations where frequent access to several different CD-ROMs is needed, automatic disk changers (autochangers) or CD-ROM jukeboxes are available.

CD-ROM was first announced in 1983, but did not become a mass-market medium until the mid-1990s.

CD-ROM (compact disc read-only memory) Optical storage device for computer data and programs. A compact disc, a CD-ROM can store more data and allows much faster access than a magnetic disk. Computer games, encyclopedias and other software are now available in this form.

CD-ROM / ˌsē ˌdē ˈräm/ • n. a compact disc used as a read-only optical memory device for a computer system.

CD-ROM (ˌsiːˌdiːˈrɒm) Computing compact disc read-only memory