A CD (compact disc[source needed]) was a digital-based storage disc used on Earth in the late 20th and early 21st centuries. Regular CDs were usually used to store recorded music, while CD-ROMs were used to store computer data.
In 1999, Grace Holloway owned Madame Butterfly by Giacomo Puccini on CD, and listened to the aria on track four whilst performing heart surgery on the Seventh Doctor. Later, the newly-regenerated Eighth Doctor had his memory of Puccini jogged upon seeing the CD. (TV: Doctor Who)
In 2002, Ace acquired a CD-Walkman from Paul Tanner, which she took with her to 1944. There, it was acquired by Nazi scientists, who developed laser technology from studying it, enabling Germany to win World War II. (AUDIO: Colditz)