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.
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            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.
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   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.
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   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.
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   Disk engine, a form of rotary steam engine.

   Disk shell (Zool.), any species of Discina.
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