A CD-R (Compact Disc-Recordable) is a variation of the Compact Disc invented by Philips and Sony. CD-R is a Write Once Read Many (WORM) optical medium, though the whole disk does not have to be entirely written in the same session. CD-R retains a high level of compatibility with standard CD readers, unlike CD-RW - which can be re-written, but has lower compatibility and uses more expensive media.