An optical disc used to store music and data. Commonly abbreviated CD. Compact discs are made from a thin disc of polycarbonate and a metal layer in the middle of the disc. They are available in a range of sizes but the most commonly available by a long way is 120mm in diameter. A 120mm disc can store about 74 minutes of music or about 650 megabytes of data.
Compact discs cannot be easily recorded; they are manufactured by etching a glass plate and using that plate to press metal. However there are also CD-R discs, which can be recorded by a laser beam using a CD-R writer (most often on a computer, though standalone units are also available), and can be played on (most) compact disc players.
The correctness of the spellings "disc" and "disk" is not trivial, see http://www.bartleby.com/61/16/C0521600.html