The Collaborative International Dictionary of English v.0.48:

Bulb \Bulb\, v. i.
   To take the shape of a bulb; to swell.
   [1913 Webster]

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.
      [1913 Webster]

   2. (Anat.) A name given to some parts that resemble in shape
      certain bulbous roots; as, the bulb of the aorta.
      [1913 Webster]

   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.
      [1913 Webster]

   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.
      [1913 Webster]

   3. a light bulb.
      [1913 Webster]

WordNet (r) 3.0 (2006):

bulb
    n 1: a modified bud consisting of a thickened globular
         underground stem serving as a reproductive structure
    2: electric lamp consisting of a transparent or translucent
       glass housing containing a wire filament (usually tungsten)
       that emits light when heated by electricity [syn: light
       bulb, lightbulb, bulb, incandescent lamp, electric
       light, electric-light bulb]
    3: a rounded part of a cylindrical instrument (usually at one
       end); "the bulb of a syringe"
    4: anything with a round shape resembling a teardrop
    5: lower or hindmost part of the brain; continuous with spinal
       cord; (`bulb' is an old term for medulla oblongata); "the
       medulla oblongata is the most vital part of the brain because
       it contains centers controlling breathing and heart
       functioning" [syn: medulla oblongata, medulla, bulb]
    6: a rounded dilation or expansion in a canal or vessel or organ