The Collaborative International Dictionary of English v.0.48:

Disk \Disk\ (d[i^]sk), n. [L. discus, Gr. di`skos. See Dish.]
   [Written also disc.]
   1. A discus; a quoit.
      [1913 Webster]

            Some whirl the disk, and some the javelin dart.
                                                  --Pope.
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   2. A flat, circular plate; as, a disk of metal or paper.
      [1913 Webster]

   3. (Astron.) The circular figure of a celestial body, as seen
      projected of the heavens.
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   4. (Biol.) A circular structure either in plants or animals;
      as, a blood disk; germinal disk, etc.
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   5. (Bot.)
      (a) The whole surface of a leaf.
      (b) The central part of a radiate compound flower, as in
          sunflower.
      (c) A part of the receptacle enlarged or expanded under,
          or around, or even on top of, the pistil.
          [1913 Webster]

   6. (Zool.)
      (a) The anterior surface or oral area of c[oe]lenterate
          animals, as of sea anemones.
      (b) The lower side of the body of some invertebrates,
          especially when used for locomotion, when it is often
          called a creeping disk.
      (c) In owls, the space around the eyes.
          [1913 Webster]

   Disk engine, a form of rotary steam engine.

   Disk shell (Zool.), any species of Discina.
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WordNet (r) 3.0 (2006):

disk
    n 1: something with a round shape resembling a flat circular
         plate; "the moon's disk hung in a cloudless sky" [syn:
         disk, disc, saucer]
    2: a flat circular plate [syn: disk, disc]
    3: sound recording consisting of a disk with a continuous
       groove; used to reproduce music by rotating while a
       phonograph needle tracks in the groove [syn: phonograph
       record, phonograph recording, record, disk, disc,
       platter]
    4: (computer science) a memory device consisting of a flat disk
       covered with a magnetic coating on which information is
       stored [syn: magnetic disk, magnetic disc, disk,
       disc]
    v 1: draw a harrow over (land) [syn: harrow, disk]

The Free On-line Dictionary of Computing (19 January 2023):

disk
disks

    1. magnetic disk.

   2. compact disc.

   3. optical disk.

   Note: the american spelling, "disk", is normal for most
   computer disks whereas "compact disc", having come to
   computers via the audio world, is correctly spelled with a
   "c", indeed, this spelling is part of the CD standard.

   (1995-07-30)