CD read-only memory, a means of providing read-only access to data for use on computer systems; the term applies to the medium in general (based on the 120 mm diameter audio CD) and to a particular instance.
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 is capable of holding about 700 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 150 Kbytes per second — than hard magnetic disks (although higher than floppy disks). Modern drives operate at about 50 times this speed, giving data transfer rates of 7.5 Mbytes per second.
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. It is gradually being superseded by the DVD as dataset sizes increase.
CD-ROM was first announced in 1983 and became a mass-market medium in the mid-1990s.