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

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