The Collaborative International Dictionary of English v.0.48:

Bulb \Bulb\ (b[u^]lb), n. [L. bulbus, Gr. bolbo`s: cf. F.
   bulbe.]
   1. (Bot.) A spheroidal body growing from a plant either above
      or below the ground (usually below), which is strictly a
      bud, consisting of a cluster of partially developed
      leaves, and producing, as it grows, a stem above, and
      roots below, as in the onion, tulip, etc. It differs from
      a corm in not being solid.
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   2. (Anat.) A name given to some parts that resemble in shape
      certain bulbous roots; as, the bulb of the aorta.
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   Bulb of the eye, the eyeball.

   Bulb of a hair, the "root," or part whence the hair
      originates.

   Bulb of the spinal cord, the medulla oblongata, often
      called simply bulb.

   Bulb of a tooth, the vascular and nervous papilla contained
      in the cavity of the tooth.
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   3. An expansion or protuberance on a stem or tube, as the
      bulb of a thermometer, which may be of any form, as
      spherical, cylindrical, curved, etc. --Tomlinson.
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   3. a light bulb.
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