Bulb

Part of speech: noun (n.)

1. 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. 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.

Part of speech: verb (v.)

To take the shape of a bulb; to swell.