A compact disc (or CD) is an optical disc used to store digital data, originally developed for storing digital audio.
A standard compact disc, often known as an audio CD to differentiate it from later variants, stores audio data in a format compliant with the red book standard. An audio CD consists of several stereo tracks stored using 16-bit PCM coding at a sampling rate of 44.1 kHz. Standard compact discs have a diameter of 120mm, though 80mm versions exist in circular and "business-card" forms. The 120mm discs can hold 74 minutes of audio, and versions holding 80 or even 90 minutes have been introduced. The 80mm discs are used as "CD-singles" or novelty "business-card CDs". They hold about 20 minutes of audio.
Compact disc technology was later adapted for use as a data storage device, known as a CD-ROM.