The Collaborative International Dictionary of English v.0.48:

compact disc \com"pact disc`\, compact disk \com"pact
disk`\(k[o^]m"p[a^]kt d[i^]sk`), n.
   a disk-shaped optical data storage medium approximately 4-3/4
   in. in diameter, which stores binary data as microscopic
   nonreflective holes or pits in an otherwise reflective
   surface, and is recorded and played back by rotation in an
   electronic device containing a laser. It is often referred to
   by its abbreviation CD. It is a type of optical data storage
   medium. Compact disks are used for recordings of music as
   well as of data for computer applications.

   Syn: CD.
        [WordNet 1.5 +PJC]

The Collaborative International Dictionary of English v.0.48:

laser disk \laser disk\ n.
   A digital data storage medium consisting of a thin disk onto
   which the data is impressed by a laser, in the form of a
   linear sequence of dots; same as optical disk. The data is
   subsequently read back by the use of a laser beam. See also
   compact disk.

   Note: The original disk may be created by use of a laser, but
         copies of the original disk may be created by a
         replicating process which does not itself require a
         laser.
         [PJC]

WordNet (r) 3.0 (2006):

compact disk
    n 1: a digitally encoded recording on an optical disk that is
         smaller than a phonograph record; played back by a laser
         [syn: compact disk, compact disc, CD]