The Collaborative International Dictionary of English v.0.48:
Bulb \Bulb\ (b[u^]lb), n. [L. bulbus, Gr. bolbo`s: cf. F.
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
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.
3. a light bulb.