Definition: CD stands for Compact Disc. Compact Disc refers to both the disc and the digital audio playback format developed by Philips and Sony in which audio is digitally encoded, similar to the way computer data is encoded, into pits on a disc. The encoding system used in the CD format is a mathematical representation of the music that is recorded. The CD was introduced in 1982 and is still in use. However, disc-less methods of listening to music, such as streaming or dowloading content from the internet and playing it back on devices, such as the iPod, are now beginning to eclipse the CD.