Noah Webster's New International Dictionary of the English Language

1. (n.) A spherical 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. (n.) A name given to some parts that resemble in shape certain bulbous roots; as, the bulb of the aorta.

3. (n.) 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.

4. (v. i.) To take the shape of a bulb; to swell.